Well in my opinion all of the preamps available on soundcards and affordable consoles ar about the same quality. If you really want to have something superior in quality you'll have to spend a lot more (about $1000 per channel.) Now to go with a mixer or an interface with mic preamps is a question of choice.
I personnaly hava a mixer as a front end to my soundcard. In a recording situation a mixer is very useful to mix the different signals, have separate headphone mixes for each musician... Plus, I use this mixer on live gigs.
To sum it up if you plan on recording a full band and need to route and monitor different signals, a mixer is a good thing to have.