I've been to both (though only one day at Plattekill) and am considering this exact trip with family this year. To piggyback on what others have said -- absolutely ski Sun/Mon instead of Saturday if you can swing it. My ideal way to do the trip would be Sunday at Plattekill and Monday at Belleayre, when it will be less crowded. However there is some chance that Plattekill will have very little open (and unless we get lucky with a well-timed storm, it almost certainly won't have a *lot* open) and it will be better to do two days at Belleayre -- the issue there is I assume Belle will sell out in advance for both Sat and Sun. So if that's your plan, pay close attention the week prior to weather, terrain status, and ticket availability.
The advantage of the ticket sales cap at Belleayre is that while it will definitely be crowded, it shouldn't be insane. In my experience they also do a good job of managing the lines at the two main lifts. But if you want to spend time on the long blue cruisers at either end of the summit, get to them early.