A preamp is more specialized. And more pricey. A standalone mic preamp generally give you a lot more versatility on one or two mic channels, but from what I've seen, a good mic preamp with more than one input will probably cost $mucho.
A mixer is basically used for signal routing; that is, plugging a bunch of line or mic inputs in and condensing them down to a single stereo output. Of course, you'll still need a soundcard with stereo input usually, though some (but not many) mixers these days also have USB output.
My advice would be to try the stuff you already have first; you got that m-audio interface and the two mics, right? See how they work. The alternative will cost more.
If you do end up sending the USB audio interface back, my advice to you would be to grab a mixer (behringer is a good low-end brand, mackie is a good high-end brand) and an e-mu 0404 audiocard.