Having installed two ******** enclosures while messing with my single 12" install, I would logistically recommend the two 10". The reason for this is that the 12" begins to get close to the edge with the mounting screws. The 10" would leave a little more room for screwing it down. I mention this only cause in doing my install I said several times "man a ten would have been much easier".
Also at Topdown, I think we focus on the sub adding because it is easier then replacing the other speakers. While your point is very valid, adding a sub it the easiest to do first. As mrray told me once, once you start upgrading your audio you wont stop.
Cheers
K