Almost any modern interface will work fine.  There isn't that much difference in preamp or converter components in the same price range.  The biggest differential has to do w/ your computer and interactions w/ the interface drivers.  99% of the time everything is fine w/ out of the box, but if you are the unlucky 1% it is nice to have a retailer that will let you exchange the hardware - no questions asked.
 
@

Focusrite works great under Linux. That is normally a guarantee for the endless support in the future as well as no general mistakes in the hardware design. I have no experience with other mentioned interfaces.

I like the UR44 a lot. Focusrite seems like a good choice as well.
The UR44 has great preamps, four mic inputs, lots of outputs, records up to 192K, will do re-amping well, works with my PC and iPad, works well with Sonar and Studio One. The list goes on...

One thing I like with the Focusrite is that you get a free version of Live Lite which gives you a cheap upgrade price. Perhaps there are other ways to get that same price, though.
 

do any of these offer 'latency free' monitoring?
 
 

UR44 uses a mixer that lets you dial in source vs. DAW. I rarely use it for source because I am getting a fast response thru the DAW. With the mixer, I can use the built in EFX to play along with YouTube videos. Sounds great.

Thanks everyone for the great advice. It sounds like all three devices are quite good and are being used successfully with Sonar. 
 
AT mentioned the value of buying from a retailer who has a hassle-free returns policy. Which retailers are the best in terms of returns and general support?
 
thanks again,
 
Paul

RDD
AT mentioned the value of buying from a retailer who has a hassle-free returns policy. Which retailers are the best in terms of returns and general support?

I like Sweetwater the best. On Amazon, you have to be careful which vendor you select from those available for any given product. Dealing with Sweetwater directly has worked great for me. Unless Musician's Friend has a killer price (like the Stupid Deal I got on the US-16x08), I stick with Sweetwater.

Sweetwater.
 
Remember they have a 2 year warranty. Most others only have a 1 year.
http://www.sweetwater.com/about/warranty/
 

Sweetwater. Remember that they have a 2 year warranty instead of just the 1 year manufacturer's warranty. Check that out:
http://www.sweetwater.com/about/warranty/