its all personal prefrence really and how much space you have, if you dont have much space then id go with 10s cause you could have a smaller sealed box, but either one will sound fine judging on what kind of subs you get
1st you'll have a heck of a time finding a box to fit the 12's in a mustang. I had to search high and low for mine. Then I had to end up going single boxes for each sub. The mustang has a huge trunk but the way they are built makes it really really hard to fit it in there. Check this aspect out before you commit yourself. If u have a buddy that has a pair of 12's, see if he will let you just try and put them in, you'll see what I'm talking about. Therefore you are much better off with the 10's, there is no problem getting them in there.