So, based on more research and the replies I'm seeing here, I'm feeling pretty good about the Focusrite 18i20. 
 
The only thing mucking this up now...is that Tascam US-16x08 mentioned by TheMaartian.  I'll be damned if that's not the perfect I/O solution for everything, in my case.  I currently have a Mackie Onyx 1620 mixer with 8 mic pre's, and I could send those analog into the 8 line-ins of the Tascam from the line-outs of the Mackie - a kind of zero latency mic pre expansion.  
 
So, for 300 bucks I could have 16 XLR Mic-pre's with all 16 channels through one interface, USB 2.  Not using ADAT means I'm not restricted to 48KHz on the expansion part, too.  If I go with Focusrite 18i20, I would have to buy another 8 mic pre box, like the Focusrite Octo, and then hook it up via ADAT, creating the sample rate limitation and all around bummerness...
 
I actually really like the Tascam US-16x08 solution, but it's Tascam...not Focusrite.  I have never ever been a fan of Tascam.  I would hate to sacrifice quality mic-pre's for the I/O advantage.  But man...I could keep using my Mackie Onyx...

Tascam started developing their drivers in-house with the US-NxN series. There have been some USB3 issues on some PCs with the US-20x20, but the other three have been solid. And I got mine on a SDotD at MF for $200! Black Friday prices are here, so I hope you can find a good price for whatever you wind up with.

Tascam still only has 8 XLR inputs so not sure what the big deal is there in terms of inputs. The Scarlett has the ADAT interface which I did not mention before so with something like the  (very affordable)  Behringer ADA8200  (which now has MIDAS Mic Pres!) it can be expanded rather easily.  (I would probably rate the Midas Mic Pres in the Behringer better than the Tascam Mic Pres too)
 
Sampling rates can be at 44.1 and 48K of course with the ADAT interface and there really is no issue recording at either of these rates either.  (bit depth is more important remember)  Higher sampling rates can be used with the Scarlett too using the ADAT interface but you only get 4 extra inputs at 96K though.
 
Focusrite has a bit of a heritage with its Mic Pres so they may be a little better. They are super nice on the Clarret interfaces though for sure.  All I know is the Scarlett is very solid for me.  Very easy to monitor input levels too.  Two headphone outs which can be used.  Also Focusrite Control is excellent and can do all manner of routing etc..
 
And also if you do get a Focusrite product they give away a whole lot of amazing plug-ins and stuff.  Like the XLN Audio Grand piano for free.  Plus some Red plugins which are also very nice.  At least once month they give away something very nice in terms of plug-ins.
 
Drivers are super solid and always very up to date.  They have even released new Mac drivers for High Sierra.  One of the first to do it.  The drivers are part of Focusrite Control so you only have to update that and it is all done.
 
 

Jeff Evans
Tascam still only has 8 XLR inputs so not sure what the big deal is there in terms of inputs. The Scarlett has the ADAT interface which I did not mention before so with something like the  (very affordable)  Behringer ADA8200  (which now has MIDAS Mic Pres!) it can be expanded rather easily.  (I would probably rate the Midas Mic Pres in the Behringer better than the Tascam Mic Pres too)
 

 
Tascam has 8 more TRS line level inputs on the back, Focusrite does not.  It's just very tempting because it super simplifies the set up - simply going analog from any outboard mic-pres.  In my case, I have 8 channels of mic-pre's already that I could just send to those TRS inputs.  And I'm done.    
 
I only record at 48kHz anyways, but I thought it might be nice to have the option to go to 96 someday.  
 
Your statement on the Behringer ADA8200 brings up a nagging concern concerning latency and how a DAW deals with it.  If the latency on the Focusrite is, say 5 ms, for instance.  And the latency on the Behringer ADA8200 was 10 ms.  Then they are connected via ADAT, how does the DAW deal with some of the inputs being a 5 ms delay while others are 10 ms (assuming the ADAT link has zero latency)?  Essentially, we have two different sets of AD conversion providing a different amount of time to do their job - so how does the DAW deal with that?  
 
I have to admit that has been hanging me up a bit.  I would rather go with Focusrite and use ADAT to expand, but I don't want to start fighting with latency disparities between these units.  I'm trying to imagine how I monitor at the interface for zero latency monitoring during recording when I've got drum audio coming in from both units.  (It takes about 10 inputs for live drum recording).  
 
 
You're making a great case for the Focusrite, by the way.  
 
 
 

Yes the extra Line ins on the Tascam could be handy for sure. RME do that too. How many extra line inputs are there though?
 
Latency on the Scarlett is way under 5mS for me.  It is more a function of the software.  I can get very fast latency on the Scarlett with Studio One.  The ADAT latency is the same for me as the Scarlett latency.  They are not different.  Not sure why it would be different.
 
I have recorded a band live (in the studio) through Scarlett (with the ADA8200 connected as well) and had a PA hooked up to a buss (in my software) so I could put anyone in the PA by simply creating a send and sending that to the PA buss. e.g. software monitoring.  There was no perceived latency of any kind. (Although I don't use any plugins while tracking though) with either Scarlett or ADAT inputs. 
 
Also the Scarlett can be used as a mixer live.  You connect it to a computer to it and create a preset that mixes all the XLR's into stereo and sends to outputs 1-2 say.  Then live, no computer is required at all.  Scarlett can be used as a 8 into 2 mixer.  Also Scarlett can be set to send all its 8 XLR Mic inputs out to the ADAT outputs as well. (instead of using the USB connection) i.e. it can be setup to be the same as the Behringer ADA8200 i.e. used as an extra 8 inputs over ADAT for another interface you may have connected.  Can the Tascam do that? I am not sure if it can but that is one cool feature. 

Jeff Evans
 
The ADAT latency is the same for me as the Scarlett latency.  They are not different.  Not sure why it would be different.
 
 

It may just be something I don't understand.  
 
Device A requires 3 ms to convert audio into digital.
 
Device B requires 6 ms to convert audio into digital.
 
Device B connected via ADAT to Device A which is connected to DAW via USB 2.  The DAW receives all digital over the USB 2 connection to Device A.  
 
Input 1 of Device B would have a 6 ms delay since that device takes that much time to process the signal.
Input 1 of Device A would have a 3 ms delay, same reason.  
 
How does the DAW know when to adjust the resulting audio 6 ms or 3 ms?  Does the DAW test each input to notice the latency is different?  If both boxes have different latency, there must be something to correct or deal with that when they share the same USB pipe to the DAW.   
 
 

I think with straight A to D conversion the latency involved is very low.  For example I have a Yamaha digital mixer and the input to output latency is quoted like something under 1mS.  And that is from analog in to analog out as well. Which is the worst case scenario.
 
So I am assuming that all the actual A to D conversion latency is well under the system latency.  That is why I think the conversion latency for both the Scarlett and the ADAT interface are probably very low and very similar.  But once that signal heads towards your DAW then you have the input and output latency of the system being added and it will most often be higher than the conversion latency.

I owned the 1818 VSL for awhile. The pre-amps were excellent, but it always had terrible USB conflict problems. If I had more than one USB device hooked up to my PC at least 50% of time it would start up with a huge feedback noise that could only be resolved if I unplugged other USB devices. I also really disliked its mixing software, which was difficult for me to understand. 
 
I've since upgraded to two generations of the Focusrite 18i8. Never had a problem with Gen 1, although you really had to crank the pots to get a loud signal. Second gen is definitely higher quality amps, but I had problems with dropouts playing VSTs and playing back Sonar files with large numbers of audio tracks. I thought my PC might be a bit outdated so i upgraded to a current gen I-7 with an SSD and 16mb RAM. Since I did it's been working perfectly and I think I'll keep it.