There is an argument made by some that the audio quality is superior for XLR. It is a technically true statement but you will be hard-pressed to find anyone with ears sensitive enough to hear it.
XLR is superior in more practical ways though. A solid majority of audio gear uses it, so putting a nice tube preamp in line with your mic is possible, or a good compressor. With USB mics, basically everything needs to be done in post. (I know you can do effect chains inside most audio recording packages, but lets not pretend those effects are superior to dedicated kit).
And the most practical reason of all? Tell me where you can find a 100 foot USB cable. With XLR, it's easy. So you can put your mic wherever you want to put it.