A week's worth of groceries (or Price Club) for that matter fits fine in the Coupe.

I believe Insurance cost would probably be slightly higher for the Coupe than the sedan if you compared a Z3 3.0 to a 530i. Of course, this could vary depending on who your insurance carrier is.

The Coupe can handle luggage with no problem, but the trunk will be full.

Functionality, the 530i probably beats the coupe in terms of trunk space, # of doors, people is can haul and interior room.

The only thing the Coupe beats the 530 in is it's uniqueness and sportiness. The Coupe is definitely a more sporty car in terms of looks and performance.

If you get the 530i, anonymity is the kind of reaction you'll get from people on the road (5 series cars are common in the Bay Area). With a Coupe, you'll turn some heads.

Now, if you've already got a car that can function as a sedan, than a Coupe is wonderful way of making a sporty statement.