dar57, Most rooms vary enough, that there are no hard rules. My room is very similar to yours, though I do have a rather high ceiling.
I prefer the long wall placement when it can work. I have my Thiel's backs out 3' from the wall behind them, 10' apart center to center, >5' from speaker cabinet wall to side wall, with my listening position close up against the opposing wall with a large piece of thick foam with thick fabric covering, speakers are about 9' from the listening position firing straight ahead, without toe in.
Of course one could use other approaches, i.e., rule of thirds, etc.. With Thiels do try to keep 8'-12' from listening position to speaker. Keeping the entertainment center cabinet well away from the speakers is a very good idea!