I've been during both weeks several times - we alternate between them so that we can get two trips out of one AP. Both weeks have very low crowds (for WDW). (If you are used to going during this period, it is more crowded than it used to be. (All the parks were virtually empty in December 2001). However, IMHO, it is now just crowded enough to not worry about any activities being cancelled or only scheduled for once or twice a week - that's a big PLUS!
Honestly, there is no significant difference betweeen the weeks. Neither Pop Warner nor Mousesfest attendees make any noticable impact on park attendance. Once in a while, we'll hit a crowd at a particular attraction or area. We just go to another attractiojn/area and by the time we return the momentary "blip" is gone.
Visit whichever week works best for your schedule.
Best wishes -