That $20 preorder bonus is a good route to get both. But i'd still say U2.
Single player is dope. Pretty much flawless. MP is great also. Variety in modes, terrain, guns, and GREAT SUPPORT!!! There have been 2 occassions where ppl lagged hella bad, but that was all. They are always taking live game feedback and changing things so it's always fair.
I preodered both at the same time, but I no longer want MW2. I'd wanna play their maybe 10hr campaign but then i'd trade it back in. U2 is more than enough for me. Oh, and I've gotten the Platinum trophy and I am still playing the single player! On my 6th run thru!