Ryan is also correct in that you should get a slightly deeper bass with a 12", however it all boils down to everything being equalized.
Ive heard perfectly matched 8's that produced the same sound as 12's that were not perfectly matched. on the flipside, Ive heard some 12's that hit so hard its difficult to hear the rest of the music, and not while listening to rap/hip hop, but listening to hard rock.
so yes, the type of music you listen too should point you to the setup.
I use 2x10's because the music I listen too needs hard/deep bass while also needing to be precise when it hits. so 2 10's are a perfect match for my musical tastes.
my neighbor listens to a lot of rap, and he is using 2x15's, which of course can be heard when he is a mile or so away...
that amp is pretty good, so you should be ok going with a single 12" but I still say if you want to stay with close to the same sound go with a single 15"
if its boxed correctly you should see the results your looking for.
Im cheating and getting ready to make a box that will allow me the use of 1 side of my rear seat while still using both my 10's