From a purely technical standpoint, the theory says that a 2x10", will behave better acoustically, mechanically, and sonically, BUTTTTT...... the subjective side of things almost unilaterally leads to 2x12".
The stupid stuff:
1. The smaller speakers CAN have their acoustic centers closer together, reducing comb filtering and providing a more predictable coverage pattern.
2. The smaller speakers have less intermodulation on their cone surface, reducing the beaming effect that larger speakers can exhibit.
3. Smaller speakers tend to sound more acoustically accurate. They have less mass, therefore reacting quicker to the incoming signal. This gives them the punch many like.
4. Smaller speakers have a disadvantage on SPL production though. They have less surface area, therefore they don't displace as much air as a larger driver.
5. Smaller speakers have less low end sound reproduction unless the cabinet they are in is tuned very well.
So while in theory, a 10" cabinet should be a sonic improvement, the neurotic nature, and subjective ideals most guitarists have, drive most to go the 12" route ( me included ). I have gotten to mic MANY 10" cabinets and I cannot say that any sounded inferior to a like 12" loaded one, only that they have a sound and it is the guitarist that makes it work or not. What I do find interesting upon reflecting on it, is that most of the bands that I have worked with ( providing sound support for ) that I have truly enjoyed and lauded, used combo's and or dual-driver cabinets loaded with 10" drivers. I would say that it was mostly due to musical genre though. I like southern rock, blues rock, and 70's rock, and most bands that will utilize a Peavey Classic 30, a Princeton, or a Bassman are predominately such the type to do so. So what I like to hear from a band versus what I prefer myself are two different things I guess.