Day One buy? Probably neither. Will probably buy Skyrim when all the patches come out (as I recall that Elder Scrolls games are famously buggy at launch.) Need to finish Dragon Age 2 and get through my second Witcher 2 playthrough as it seems that Skyrim will suck my soul for a good long while.
MW3? I'll probably borrow it after borrowing MW2, MW, BLOPS. Not really a shooter fan (as I'm only now working my way through HL2 and waiting for DNF). And given a choice, I'll probably buy BF3. Really enjoyed 1942 and BF3 has jets and tanks XD
Off topic I know, but Skyrim over MW3.