I'm currently deciding pretty much the same thing. My bedroom may be too small, I think. A little smaller than yours ( ~10.5'x12'). The L420's were stunning. The L520's were even more so. Now I am trying out the (German made/Braun M9's, M9/90's when I finish the crossover).
This is a crappy answer, but- all of these speakers I've tried in here were great. It really depends a little, I think, on how flexible your room is to speaker placement, where youre going to be listening, etc.
All of the above models were absolutely excellent in my room, and I still have all of them. Still trying to decide which is really best. I could really move the larger models out to the living room. Both the L420's and L520's were great. I even tried stacking them, which was pretty damn cool as well. Right now the current rotation has the M9/90's playing in my room. I just elevated them about 2' 3" off of the floor (as opposed to directly on the floor, aimed slightly up) and they sound wonderful. So really, like you- I don't quite know what to do either!
Really though, I think that the L420/L520's are absolutely wonderful for rooms of this size, though the larger models can obviously be great too, if you are willing to give up the space... In my humble opinion, you may be more than satisfied with the L520's in your room- why not just put the other set in the living room? : ). Again, I have a similar dilemma, so this rant may not have been very helpful. I need more time for testing, DAMMIT!