I would go for 12,000 BTU Mitsubishi, get an "Inverter" type, more expensive initially but will pay back quickly as far cheaper to run.

Advantages:- (they reckon!)

At least 30% - 50% cheaper to run as it consumes less power
Far quicker to achieve desired temperature
The start up time is reduced by 30%
Much quieter
No temperature fluctuations, maximising comfort level
No voltage peaks from compressor.