Looks like you have one serious dilemma
. All though I do drive my FX more because of it's practicality, if given a choice I would definitely stick with the M3. The exhilaration of going from 0-60 in under 5 seconds can never be matched by the FX even though there is a 20hp difference.
If this is going to be a designated family car, then the FX would be a more viable option. Contrary to its sporty looks, it has about the same luggage capacity as an X3. And with it's 5-speed automatic transmission it does get descent gas mileage for a V8 car.
However, for short office runs the M3 be a better choice given its more fuel efficient in addition to it being a more fun car to drive. Either way keep us posted on your decision and if you have any specific questions feel free to let me know.