My view is that first you should decide what fits in your car. In some cases two 10s cans fit in places where a 12 can't (for example, next to wheel wells, resulting in nice stealth install). If you don't have any limitations with regard to the size of boxes, etc, one argument against the dual 10s is the value. Two 10s will have a bit more cone area than the 12, so they will have slightly better sensitivity, but at a hefty cost. Usually, there is not much price difference between a 12 and a 10 inch subwoofer from the same model family, so dual 10 setup costs almost twice as much. For me a single 12 works, so under normal conditions I would prefer to buy one high end 12 instead of two cheaper 10s.
A more interesting comparison would be to compare a 10 in a ported box vs a 12 of the same family in a sealed box. The sensitivity should be comparable between the two. The ported box for a 10 will be bigger than sealed 12 but not by much.