Consider both sides:

With 10's you'll have tighter, probably cleaner bass and the box will more than likely take up less space. However, you may feel like you want more bass later on.

With 12's you'll have deeper, more powerful bass (compared to an equivalent model as the 10's). They'll also be more than enough assuming you put the right power to them, and you can always turn down the gains on your amp if it's too much. However, the box will be a bit bigger and take up more room.

It's pretty much up to you...what kind of bass will you be looking for? I've always thought 10's were plenty, but I've also always preferred 12's just because I'm weird like that...