From what I know of the two bikes, the CBR150R and the CBR250R, the 150R is a lighter, shorter version of the 250R, but have a higher ground clearance. The fuel tank is also same, so width wise, there shouldn't be any difference. I haven't driven the 150R, but the 250R is not good for me, as a few hours of ride made my back sprain.
The back pain you are getting may be due to your height(I believe you have a short hands which made you to bend to reach the handle bar).
If you get a chance test drive TBTS, 150R and 250R back to back in city and highway. And you will know what you want.
Some of the 150R owners have reported a mileage of 45+(when kept under 5k rpm). Choose 150R if you want mileage or choose 250R if you want power or choose TBTS if you want pride-of-ownership & a torquey engine.
@d3p: If i get a chance, I will surely test drive it, Dude.