jbraner
Larry - I think you should pay serious attention to the rt latency (like you are doing...)
Everyone is different - but I bet you really don't want to go much higher than 8-9 ms, maybe 10ms at the most.
 
"zero latency" monitoring is useless for people like us, who want to hear the signal after in goes in to SONAR, through the amp sim, and back out again ;-)

jbraner (and everybody else) - How about this idea: I split the output of the guitar and plug one into an external FX box such as my Pod then into the interface, and the other directly into the interface. I record the dry guitar signal in Sonar. The Pod signal I use only for monitoring, the Pod providing me with an amp sound so I can get a feel for my playing.
 
I don't know exactly how this would work technically, or if the various driver/mixers I'm considering would be able to handle this routing, but if I could make it work I could buy any interface and not have to worry much about rt latency.

Larry, I would say it makes more sense to just get the latency as low as possible and play to the actual amp you're using.

Maximum - I feel exactly the same as you and will probably go with the Ultralite AVB in the next month or two. I'll be sure to report back - maybe in a new thread. I made sure if I buy it via Amazon that I can return it within 30 days if I can't get the low latencies that I'm hoping for.

mixmkr

Rimshot
The EFX are VST's that can be applied to the real time monitoring and are also available thru your DAW.
You cannot apply these EFX to the signal going into the DAW - only after. 
 

Actually you can record the FX...
Here are a couple of videos I did, back when I got my UR-44.  Yes the headphone outs are a tad low if you like them REALLY cranked...and the pres really not enough gain for good ribbons or some other low output mics...but quiet for that price range.  In those situations, I run another pre-amp before the interface.

https://www.youtube.com/watch?v=eTR75d-Oz9g  (FX and GUI)
https://www.youtube.com/watch?v=BlxxTnFGRtk  (Guitar amp sims)

mixmkr -  Can you monitor with the amp sims and not record them?
PS: Videos are great! Highly recommended.

I can get my Scarlette 6i6 down to 8.5 ms RTL @ the setting of 5ms , not sure but I think that translates into what most people call 128 samples? ) and my system is stable if the project is stripped down and I just run something like Guitar rig. And I'm using 44.1. It guess it would perform better at higher clock rates? Below 5ms I have issues with dropouts. 

For an interface with that mix capability, I would assume you can send different mixes to different outs easily. Two headphone outs is needed for such. In Focusrite's mix control, analog 1 is different from FX1, and I would think the UR44 is similar (if not almost identical).

Cactus Music
I can get my Scarlette 6i6 down to 8.5 ms RTL @ the setting of 5ms , not sure but I think that translates into what most people call 128 samples? ) and my system is stable if the project is stripped down and I just run something like Guitar rig. And I'm using 44.1. It guess it would perform better at higher clock rates? Below 5ms I have issues with dropouts. 

Cactus -  What the picture shows is my driver settings from the Sonar Preferences dialog. Sampling rate: 96000. (I have that same rate set in the ASIO panel, with a buffer size of 256 samples.) In case the picture doesn't appear,  the ASIO reported latencies displayed in the Sonar Driver Settings are:

• Effective latency at 96k stereo: 2.7msec
• Input: 3.1msec 299 samples
• Output: 3.0msec 284 samples
• Total roundtrip: 6.1 msec 583 samples

This gives me no dropouts or instability, and undetectable lag when monitoring my guitar through Sonar with an amp sim. My projects typically are around 30 tracks, including sampled drums, synth bass and a couple of keyboard synths plus vocals. I don't know how these settings translate to what you see in your Focusrite box. I'm just looking for a USB box that is as capable as my old inexpensive sound card.
 

mettelus
For an interface with that mix capability, I would assume you can send different mixes to different outs easily. Two headphone outs is needed for such. In Focusrite's mix control, analog 1 is different from FX1, and I would think the UR44 is similar (if not almost identical).

mettelus - You'd think so, but the Steinberg site is quite vague about exactly how this works in their interface.

LOL, 96K is "cheating" I never record at that, but finally have the system back online so can give you quick numbers, 96K/256 buffer is 733 samples total (8.3ms), 44.1/128 is 367 (also 8.3ms), and 44.1/64 is 239 (5.4ms). It will go down to 32 "supposedly" but never tried it. As Jim Roseberry mentioned, I assume there is a "wall" at around 5ms anyway (and the CPU load is unnecessary).
 
From Johnny's thread on offsets, it seems the Focusrite products are more optimized to the 48K spec. Another thing to bear in mind is my offset is 76 samples - it seems this is "built-in" to accommodate loopback functionality inside the box.
 
I may have missed it in the thread, but which amp sims are you running?
 
Your one comment made me chuckle. Personally if it isn't broke, I don't try to fix it (but planning is good). You are "banking" on impending doom, but worry like that is best dealt with when the time comes.

mettelus
LOL, 96K is "cheating" I never record at that, but finally have the system back online so can give you quick numbers, 96K/256 buffer is 733 samples total (8.3ms), 44.1/128 is 367 (also 8.3ms), and 44.1/64 is 239 (5.4ms). It will go down to 32 "supposedly" but never tried it. As Jim Roseberry mentioned, I assume there is a "wall" at around 5ms anyway (and the CPU load is unnecessary).
 
From Johnny's thread on offsets, it seems the Focusrite products are more optimized to the 48K spec. Another thing to bear in mind is my offset is 76 samples - it seems this is "built-in" to accommodate loopback functionality inside the box.
 
I may have missed it in the thread, but which amp sims are you running?
 
Your one comment made me chuckle. Personally if it isn't broke, I don't try to fix it (but planning is good). You are "banking" on impending doom, but worry like that is best dealt with when the time comes.

mettellus - Yeah, I've been cheating since the beginning. I got into Sonar by myself, and my whole learning process was setting it up (by myself) and recording & mixing (by myself), so I had no one to tell me that 24/96 was "too much." After all, the sound card is actually called "Audiophile 2496." How was I to know? But it has worked well for me: Until I started shopping for a replacement, I didn't even know latency was an issue.
 
As for worrying too far in advance, Part B of my plan is to start using my laptop for recording on location, so that's what got me started looking for a new interface. Worrying about drivers for Windows 10 was just a fun bonus.
 
Thank you so much for all your help!
PS: I use Guitar Rig 3 (that came with an earlier version of Sonar), TH2 Producer and various guitar FX chains by Craig Anderton. I just don't want to mic an amp and make a whole bunch of noise, and most of my music doesn't really require that sound anyway.

I almost replied to the Thunderbolt thread (that would have confused everyone!). I know GR4/5 have a "HI" CPU button in the upper right, which would allow smaller buffers to help with most any interface when off. I "assume" GR3 has this, and not used TH2 enough to ever notice what/where this is.
 
When the time comes, be sure to get something from a place that has a nice return window so you can run whatever you get through the ringer to make sure you are satisfied.
 
If you do happen to choose Focusrite, this tutorial is very well done on how to use the "MixControl" software. It will ease the learning curve tremendously.

metellus - I'm messing around with my current setup trying to increase the round trip latency and see how much it screws up my playing. It occurred to me yesterday that we are talking about milliseconds -- thousandths of a second! Intuition suggests that I may be freaking out about a time lag that I would not even notice. I mean, Sonar is showing me an RTL of 6.1 msecs and others are saying this or that device yields 11.4 msecs or whatever. So yeah, it's almost double, but we're still talking about just five thousandths of a second.