Well the obvious is the 4 inch of cone loss. The 12's can handle the lower bass verses the 10. The power in some cases may increase or they may be rated the same, but if your only swapping out the subs and keeping the same amp, you may notice a BIG difference or a slight difference. Depending on the box size etc. alot of things can change.
well i can't give you as smart as a answer as him but most likely you would get better and louder sound from 2 10s than one 12. but are you using this for comps or just to play "real" music. a 10 can easily handle the bass line in real music because the bassline rarely goes below 60hz because they have to make the cd playable on a variety of audio system setups (stock system, boombox home stereo, walkman ect.)