The COLT, in a hot second. There are millions of 870s, and they are still made. Not quite dime a dozen, but you can find them everywhere. Not so the Colt.
and, its already a collector's gun...not the high demand high dollar kind, yet, BUT they aren't making any more of them, its value will not decrease.
All else being equal (and it almost never is) bigger bullets tend to work better.