There are a few people on this forum that have owned/own MINI's, so do not be surprised. I think it will come down to what you want to do, from what you posted earlier about mods, the MINI is one of the best vehicles on the roads to do modifications to (many vendors available and not very expensive)
. There are not many Z4 modifications available, and the ones that are, tend to be more expensive than a regular vehicle.
For handling and driveability, take them both out for a test drive and decide for yourself what fits your desires.
Cheers!