Tough, tough choice. My choice would be the M1. I have both, and the AR15s get shot a LOT more, because they're fun, and ammo is cheap.
But, I won't part with the M1. It's a Winchester receiver, built in 1942: a WWII veteran. I hold it and wonder if some young Marine hauled it ashore on some God-forsaken tropical island, or if a young soldier struggled up the beach at Normandy with it. Don't try to convince me otherwise. Oh yeah, and:
BANG! BANG! BANG! BANG! BANG! BANG! BANG! BANG!-piiinnnngggg! An experience all its own.
It's one of those rifles that get attention every time you go to the range.
My AR15s are homebrew, and there's a lot of pride in shooting a rifle that you've assembled yourself. They are FUN to shoot. I haven't gotten the accessorizing bug, at least not too bad, but that's a fun part of AR's, too. 20 and 30 round mags are common. Where I will shoot at most 40-48 rounds through the M1 on a visit to the range (shoulder gets a bit tired...), I have to limit myself to a couple hundred rounds with the AR.