As someone who has ordered a new Z4 I think I might be allowed to comment.
I have to carry round a large Samsonite suitcase for work. The first thing that I checked before any thing else was whether the case will fit when the roof is down and it will. Granted, it cannot be put into or out of the boot with the roof down but I don't see that as a problem as I will not be leaving the car parked with the roof open. I can leave my house, put the case in the boot, put the roof down and drive to work. When I get there the roof goes back up, case out etc.
If you really want to put stuff in the boot with the roof down then add the comfort access option which lifts the roof out of the way but does not put it all the way up.
Both the old and the new are good cars but they will appeal to different people. I don't like ragtops due to the lack of security but equally I'm not a hairdresser so the SLK is out. BMW have finally made a car that fits what I want so I am happy.
Enjoy your Z4 regardless of age!