As an owner of a 2000 Boxster S, I would still vote for the Porsche. Yes I am nervous about the IMS problem but sometimes these things are blown a little out of proportion. There is a fix available, part costs about $500 and it is a good idea to fix it if you do a clutch job as you are allready in there. As for fun to drive there is no comparison between the Porsche and BMW. Two different cars, one more sports and handling oriented and one more grand touring. Your choice of what you want. Also you do not need to drop the engine for any service that I am aware of. I have changed oil, air filter, spark plugs, and belt by using the axcess holes on top and behind the seats.
Ps. Did I mention the sound of the Porsche, especially if you remove the intake snorkel.