I am in the market for a new interface.  I started with a simple Tascam US 4x4, but will be outgrowing it very soon.  I do no vocals or guitar, I am simply connecting multiple hardware synths to the AI. 
 
I had thought about the Tascam 16x08, but I've read several folks have had problems with this interface (including a long thread on this forum).  I've kind of narrowed it down to the Scarlett 18i20 or the Behringer UMC 1820
 
Both have 8 usable inputs.
 
The Scarlett seems to be well regarded, with current drivers, good support.  Downside is that it is $200 more than the Behringer.
Behringer is substantially cheaper, but kind of an unknown with Sonar.  
 
I need as many inputs as possible and want a rack mounted unit.
 
I'm sure we have some Sonar users here who have first hand experience with both units, so I would like to solicit opinions.
 
Thank you.

more expensive,
but a step up,
would be a RME UFX
 
 
OTHERS:
tascam celesonic
motu 8pre
steinberg  ur824

I went with the Behringer XR 18.  I believe it has the same preamps, but is only 48K 24 bit.  It does not got to 96k.  I typically record in 48K 24 bit anyway so it is not a problem for me.  The added benefit is that this box is also a digital mixer as well as an interface to the PC.  A Lot of flexibility.  I have not use the mixer with a band as a mixer, but it does work well as an interface!  Note price has dropped since I bought mine.  They are about $600.00 now.
I paid $700.00.  This is a USB interface (I am using USB 2.0).  It also has monitoring capabilities that are really nice as well!

This may not be much help as I've never used either of those specific models...
I've owned both Behringer and Focusrite products before, and the Behringers have always felt cheaply-made and have worn out/ broken fairly early.  My Focusrite interface is sturdy and reliable.
Perhaps Behringer build quality has improved in the last couple of years, but I'm very wary of them.

There have been comments, that I've read on forums, that the Behringer's do not have the lower latency like some other brands, and less support.  I have some Behringer equipment - an X-Touch and a Power Play Pro 8 headphone amp and both have worked well, but they are not recording interface units.  I have no idea if there is any Behringer support for either of these if a problem developed.
 
OTOH I have some Focusrite equipment - an ISA One preamp and a 6i6 2nd generation (my backup interface unit) and both of these work well, and there is Focusrite support.  What dealings I've had with Focusrite support has been prompt and helpful.
 
 

The issues the person was having with the Tascam ( I was part of that thread) were seemingly more a system issue than a Tascam driver issue. Not everyone has problems with that interface, but still I don't blame you from shying away. Behringer, even though I know they may have improved, is the only equipment I ever bought that totally failed on me before I got my money's worth, so sorry, not going to ever trust them again. 
 
I've narrowed my search down to a Motu Ultralite Mk4    http://motu.com/products/proaudio/ultralite-mk4  
It's a price jump but I think it's time I got out of using less than perfect equipment. 
 
RME is another company I would go with. 
My Focusrite 6i6 is a great interface but does not have enough inputs for recording drums or a band. I still use my old Tascam us1641 for this without issue. Both suffer so, so pre amps and not great RTL performance. 
I'd like to take the next step and do it right, so Motu and RME is where I need to go. The Motu unit would also serve double duty as a live mixer with remote control capabilities.
 
If all you want is more XLR inputs for recording drums you cannot beat the price of the Tascam's. 
 
 

Thank you all
 

I used to dislike Behringer quality but my X-Touch is very well built.
 
I have several Focusrite Scarlett interfaces including the 18i20 1st gen. and all work great.  Support is excellent.  Be sure to get 2nd generation models.
 

The Behringer unit is using the same tech that you'd find in their digital mixers.
Quality is far better than what many folks typically associate with Behringer.
Behringer has absorbed some top tier companies/people/technology (Midas, TC Electronic, etc).
 
The Behringer UMC-1820 will sound/perform reasonably well.
Lowest possible round-trip latency isn't the UCM-1820's forte.
 
Between those two audio interfaces, I'd go with the 2nd generation Focusrite.
 
If you've got the budget, I'd recommend having a look at MOTU or RME.
RME is more expensive, but it's the type of audio interface that'll last a decade... and you'll never give it a second thought.

Well, I didn't plan this to happen.  Dropped into the local Guitar Center today, and somehow ended up walking out the door with the brand new Focusrite Scarlett 18i20 2nd Gen for $360 out the door.  Was $450 at SweetWater, so I feel like I got a pretty good deal.
 
Got it hooked up tonight, and most things are working, except playing back audio tracks.  The Focusrite is set correctly in the preferences, but still no joy.  I'm sure I have something set wrong, I'll go through the manual tomorrow AM.  I'm just new to all of this.  This is clearly an upgrade from my Tascam.
 
I see folks report their latency - can someone explain to me how to do this?  Is it done within Sonar?