I would go for the $100 mic and $100 preamp any day over the USB one. For one thing, you could always get a different microphone or preamp down the road and try different combinations of mics and preamps to get different sounds. If you are serious about recording, you will want to have several mics, so having a USB Mic/preamp combination will limit your options.
My experience with USB Mic/Preamp combos has been that the sound is OK, but much better quality results can be achieved with a dedicated preamp.