Please don't compare apple with oranges. The 1140 and the 1250 were designed differently (other than looks and weight). Like it's predecessors, the 1250 was designed for wide-open spaces deployment hence the transmission footprint is significantly wider. The 1140 is designed for office space use hence you don't need a very strong transmitter.
If you upgrade the 1250s IOS to 12.4(10b)JDA3 you should be able to power the 1250, with both radios, on 15.4w but at a cost of lower MCS rates.