I have tried both, and with bad results for the FireXon (on Lion that was). The 610 on the other side works perfectly well. It doesn't shield well for noice, and the gain pots caused some crackels, but it worked perfectly fine. With my somewhat limited ears I could not find anything to complain over for the audio quality either.
My general rule for iCON btw. would be to stay away. I got a whole home studio pack in return for a favor to my local music shop because the norwegian supplyer cut the brand. And I must say Im pretty dissapointed with the lot. The only somewhat decent thing i got is the ReoTube preamps. (I never got response from the support either, when asking for working OSX drivers)
But if youre looking in this price range you should definatley consither the TC Electronic Desktop Konnekt 6. You only get noe preamp, but a good one. And you would probably want to do vocals and guitars I seperate takes anyway. (To prevent bleeds, get more steady tracks, and so on) It has a stereo 1/4" instrument input also, but don't go there. Use a mic on the guitar if youve got one.
The profire is rather low value for the money (but rock solid driver-wise). It's pretty much a go-to interface so all the smaller businesses really set it as a benchmark. They don't sell the Icon FireXon in my parts, which I see as a very bad thing as I live in London. We're not exactly remote and have lots of resellers and are one of the music hubs of the world here and the fact that nobody will stock them even online is pretty damning.
Have you considered the Echo Audiofire 4? The reviews seem pretty stellar and I know quite a few Echo based Mac users.