I wouldn't worry too much about the additional cost of the car as a deciding factor if you can afford either. Reason being that the F30 will retain it's value since it is the newer model. Especially as BMW goes towards the new style headlights on multiple models.
Something to think about if you go with the F30. Its trickier to fit wider rims/tires in the back. Where as with the E9x fitting a 10" back there with 275 wide tires was easy, you have to play around with the Camber and suspension to get a 10 to fit back there.
Also, Id consider the potential for greater aftermarket support in the suspension areas. I'm not sure how far down the line you plan to upgrade, but the larger following of X-Drive 3 series in North America is going to push the aftermarket providers to offer more options as far as suspension. Ask any E9X X-Drive guy, and you'll see that your options are very limited.
Its a good question with both cars having their own merits and negatives. If its primarily about handling then drive drive drive them as much as you can and go from there!