From what I can tell, the 15s are a few db more sensitive, can go a bit lower and can also be useful as high as the 500hz and beyond. I would guess that quad 10s would perform fairly similarly to dual 15s, but take more effort to get tweaked to perfection. I like the idea of starting with duals of the larger woofer in this case (bigger isn't always better) and having the option to expand without a complete re vamp. Once one has 4 10's incorporated, you are going to move to 8....I think
In Chrapladm's situation, it may be that room won't allow anything beyond the 8s he mentions, or maybe the 10s, either of which would probably do very well. When we first started discussing this project, someone brought up the idea of quad tens (may have been Mark Seaton).
I just modeled the quad 10S vs dual 15 and it would seem that the dual 15s have an output advantage most of the way through range we are talking about...(I can't get the chart attached for some reason) The output advantage seems significant from 100hz down. I could be wrong in my assumptions, but it seems this way from what I can tell.