2 REL Storms or 1 Stadium?

Having owned everything from Strata III to Stentor III, I would say unequivocally that the Stadium III will blow away two Storm III's. It's not a matter of quantity but of quality. To be honest, the Storm III was my least favorite of all the RELs due to its obvious bass reflex loading and the terminal muddiness that comes with it. The Stadium III is a major step up in fundamental sound quality, something you will not get by adding another Storm III. Of course, be prepared to spend more time in placement setup since it is harder to get one sub to load the room smoothly than two.

A couple comments:
1) Don't worry about room size, either will work fine in your room. You're going to have to spend some effort on placement, though, since your room is pretty close to square. If it has 8' ceilings, that's a pretty bad situation for standing wave problems as it is almost exactly 1:2:2. This will cause problems in the lower end of your satellites as well.
2) IMO, the Stadium is a bigger step up from the Storm than the Stentor is up from the Stadium (law of diminishing returns begins to kick in here as everywhere else). This is not to say that the Stentor isn't worth it; I think it definitely is. But the Stadium is still a very good sub.