Most superbGem 1976 (P) quarters are in the 1976 set. Nice Gem '76-D half dollars are a little more common in the '75 set. The '75 plastic yellows, cracks, and damages the coins far more than the '76 plastic. You can differentiate '75 and '76 sets from 10' away by this means. The Philly type I Ike often can't be restored to pristine condition because it is so porous but oddly enough the ('75) dimes can.
I can't think of any other significant differences. Grade distribution between the two dates are otherwise similar across populations.
Gems and varieties are made in batches and mint sets were distributed by zip code. This means the '76-P quarters were probably mostly made by consecutive strikes by a single die pair (or two) and they went into '76 sets. The better '75-D half dollars were probably just better process control. Other differences are systemic.
Time don't fly, it bounds and leaps.