Its only what, 20 HP difference? If she wants a manual you need the 2.4L, 2.0L is not offered with anything on any of the MKs or Caliber platforms with a Manual. That being said, I have a 2.4L Caliber R/T and enjoy it, on open highway stuff I can get 30 or so from the EVIC, and have gotten as high as 32 with mine. Probably can stomp the socks off of it with a 2.0L, but you never can tell. Mopar= a rough translation of More Power, so, being that type of guy, I got the 2.4L for the extra HP and torque. You won't be on teh skinny pedal as much with it, so I figure that SHOULD help gas mileage a little. As far as the best gas mileage goes, the Caliber SE and SXTs with the 1.8L manual get 32+ I have seen one guy on a caliber forum end up with 36 MPG from it. Honestly, I think she has it right with a Jeep over a 'yota or Honda.
Like I said, more power=more better!
2.4L(auto, available in manual):
173 HP @ 6000 RPM
165 @5,200 rpm
21 city, 25 highway (getting far better than these!)
2.0L (auto only):
150 HP @ 6400 RPM
141 @ 5,200 rpm
23 city, 27 highway
1.8L (manual & Caliber only):
143 HP @ 6400 RPM
125 @ 4,400 rpm
24 city, 29 highway (again, know of a guy getting upwards of 30, 36 was his best thus far)
I'd also like to have the power and never need it than need it and not have it.