Hi there. If you're looking at recording good quality vocals for songs or even voiceovers, you might need to set your aim a little higher than a USB mic.
I have only had some experience with using a USB interface (MBox) and listening to/mixing other peoples' work recorded through various USB-powered interfaces, and there is audible noise added to the signal when the device is powered by USB.
I can imagine that the USB powered mic would be no different.
Brother Rat also makes a good point in that a standard XLR mic can be used anywhere connected to any device with a standard mic preamp and will therefore give you more flexibility further down the track when you have updated your studio gear.
Just my opinion, but the Rode NT1a I have found to be the most versatile condensor mic in my modest arsenal, and it didn't cost me a week's wages!
Dags