+1 on Avatar. Even though there are other good manufacturers, Avatar makes some really good stuff for a great price. I have a 2x12 Avatar Vintage cab that is great. I installed casters on it and its a great cab. They also have a dent n scratch section with really good deals. You can always get one from ebay as well.
I'll join in the Avatar choir. Last night I had occasion to open up my Contemporary 1x12 for the first time and saw a -very- clean build and good plywood that made me feel a lot better about not having bought a more expensive cab. I also have a Heritage 2x10, not quite as heavily built as the Contemporary, a little easier to carry. They both sound great.
If you don't want an Avatar and are near a major city, Craigslist can be your friend.
To address your actual question: I don't have a 2x12 -or- a 4x10 but having paid attention to Bassman owners, they seem to prefer the 4x10. I will say that a 4x10 will actually give you more square inches pushing air than a 2x12, just as a 2x10 has more area than a 1x12.