I have been doing some comparisons among audio interfaces (USB) and I think I have narrowed things down to the MOTU Audio Express and the Focusrite Scarlett 6i6.
 
I know a lot of folks here have the Focusrite stuff, which means it probably works great with Sonar.  I can't find anything here regarding the MOTU.  It all really comes down to drivers, I don't want Sonar to crash or the PC to lockup.  Also, the MOTU is ~$140 more than the FS 6i6.  I am not sure the software extras in the MOTU box warrant that price difference, but the routing flexibility is nice.  I am not sure what I would do with the extra phones jack on the FS 6i6.  I don't need the plug-ins or Lite DAWs (I have Sonar after all!), but the mix software and some the the analysis tools in the MOTU kit look quite useful.  I also like the front panel meters on the MOTU.
 
Your advice/opinions are welcome. Happy Friday!

Here's a good thread to review first...
http://forum.cakewalk.com/New-Audio-Interface-Suggestions-m3016243-p2.aspx
 

wgdevanna - I have a question for you regarding what you plan to record. Are you a singer songwriter type artist or do you plan to record groups and so on?

Hardware wise, the MOTU is feature packed with both USB and Firewire, it's expandable and has a foot switch input.  However, one thing the Focusrite has over it is a second headphone output as you mentioned.  If the MOTU works well with SONAR, then I would go that route.  I think the extra $ is worth extras.
 
Kind regards,
 
tecknot