Personally I always go for more power, you can get better mpg by not using all the power available / stretching the engine. No idea if valid these days, but my 1.9 205 gti back in the day was better than the 1.6 gti I had before it from an mpg perspective.
Not sure of the weight difference between the engines on the MX, but having had mk1 and 2 1.8 engines, the fun is at much lower speeds. Got taken out in a mk1 1.8 last week & felt so much fun, glanced at the speedo to see we were doing 60mph - the same feeling in my Z4M takes beyond legal speeds. Seems that 40-50% of maximum velocity is where it starts to feel 'fun' in a non scientific way.
If the 2.0 gives significant torque increase, it will 'feel' faster & more relaxing to drive briskly. My JCW F56 feels faster than my wife's 3.0 E89, but it's at least a second slower to 60 mph - it's about how it gets delivered & feels. Drive them both & see what you think.