This brings back memories. Many years ago, I spent about a year going back and forth between the two. It was generally ok, though there were always those times when the rules from one game would slip over into the other. Hiding behind cover in USPA matches or dropping loaded mags in IDPA matches, etc. Always good for a few laughs (and the occasional PE, in the case of IDPA) during those matches.
In the end, if you want to be the best you can be at one of them - just focus on that one and then figure the other is just trigger time/practice under pressure.