Pop Warner had a HUGE effect on the crowds at MK when we were there in early Dec. 2004. Don't know which day we went to MK, but something like Dec 8 or 9. The first hour was fine, and then the lines were generally pushing the half hour plus mark (the line for Mickey at Toontown was over an hour), so we ended up leaving and going to another park. The other parks were quite empty, in contrast!
I've gone to WDW several times in early December, and this was by far the most crowded I've seen MK in a long, long time!
Since you want to spend both days at MK, I would recommend going after Pop Warner rather than taking the chance of hitting heavier crowds!