SW MP15 or Colt M4 Carbine?

[synergy]

Colt's have the larger pin size making them unable to fit other uppers, correct? Less options when you have a colt in my opinion.

False.

Their FCG have larger pins sizes...but virtually every aftermarket trigger is made in that size, too.

SOME commercial Colts had the larger take-down pins. But, there are adapter pins to get around that, too. Colt Defense guns, like the 6920 use standard size takedown pins, and there are no issues putting it on a different lower.