This will probably come as complete and utter shock here on" The Z4 Forum", but....
Threads like this are pointless - you're talking about three completely different cars, the one you pick will be the one YOU like best after much consideration and a few test drives.
The Porsche is the "race car" of the group, the other two are beautiful cruisers, but all have their strengths and weaknesses - characteristics only the OP can determine for himself.
So, in three years, or whatever, take some time and explore.
Many (especially those of us that own the Z4 and drive it) would not consider the Z4 a "cruiser". An example of a "cruiser" would be the Infiniti G Convertible. The gap between the handling characteristics of the Infiniti and the Z4 is 5 times wider than the gap between the Z4 and Boxster.
...and the Z4 transforms into the sportscar it is supposed to be after one replaces the OEM runflats with proper performance tires.
The new (2013) Boxster S is... well it's incredible. The redesigned exterior (IMO) is fantastic. The top of the car is a lot more flat than it was in years past and the front end is a little more squared off. I'd always thought that they were too rounded for my tastes, but the new one looks like a damn supercar. It drives like its on a rail, daring you to push it harder into turns.
Unfortunately, they're also more expensive and you're unlikely to see the deals that you will from BMW.
I love the Z4. They're (IMO) more pleasant inside, particularly with the added tech (comfort access, iDrive, etc). Spec'd out, they are nearly identical from a performance perspective. Hard to beat the hard top.