There it is.
GV Connect has more Google Voice features. For example, blocking callers, adding notes to your call log, changing your VM greeting, etc. Also, GV Connect allows you to search your SMSs, Voicemails, and call log.
GV Mobile, on the other hand, has... uh... a nicer looking SMS compose view? (It doesn't tell you how many characters you've used of the 160, though, like GV Connect does.)
I'll probably use GV Connect more often. Doesn't hurt to have a backup though.