I have not downloaded Mothership Zeta, and don't intend to. For me, the locations in Fallout 3 that I dislike the most are vaults. All the corridors look the same and I lose my bearings so easily. From what I've read, MZ is just a long series of vault-like corridors shooting comedy generic aliens, with minimal questlines. Sure, the weapons you can get there are good but that's about it.
I voted for The Pitt. I've done this and absolutely loved it, even if it is a bit bleak in places. It has a great main questline, new weapons, characters, equipment and enemies. Also some morally grey decisions will need to be made. Its also got one of the best energy weapons in the game, a weapon that I've only recently started to truly appreciate - the Metal Blaster.
You know what you need to do.