For the XL, it really does not matter what format you use. Both are about the same size and contain roughly the same infomation. Of course, if you are wanting to edit some of the samples, I would suggest using .wav files since you would not have to convert the sounds while on computer.

For the former model, you were limited to using .snd files if you wanted them to load as part of program. The MPC's do not store the filename extension inside the program file and the former model would only load xxxx.snd files when loading the sounds as part of a program. The XL does not care and looks for either one. Most all of this is probably transparent to you and you probably hasn't noticed the slight change in behavior.

As far as the tachnical aspect of .snd files, they contain all the identical sound data as a WAV file just in a different order. So there is no audio loss or gain by using .snd or not.