I've honestly never heard any competitors saying that they'd NEVER use soy protein during a prep. But most commonly, people in comp prep will stick with primarily whey isolate. And generally, people in prep don't use protein supps as much since A) we'd rather have real food most of the time since our portions get smaller and smaller and B) most people cut out all protein supps at some point during prep, so it's best not to get used to having shakes all the time.

Now, the issue of bloating due to protein supps... Some people cut it out during their whole prep, some cut it out during the last month, some only cut it the last week, and there are others that never cut it. But really, the amount of water that PP would make you hold is a pretty small amount (well, with whey at least).

I've never been one to use soy products much at all, so as far as the differences between the two, especially in terms of contest prep, I can't really say.