After owning multiple BMWs, I don't enjoy dealing with exploding cooling systems, and odd-ball electrical system surprises anymore, so the Z4 was off my list. People who are into S&M and odd kinks come in all forms, including automotive enthusiasts. So if that is your thing...
I can't speak to cost of ownership on a Z4 but I would imagine it would be similar, if not, a bit more than Boxster ownership. For what it's worth, my Boxster and 996 were the most reliable vehicles I have owned. The BMWs I owned previously owned were just money pits, and they weren't anything special, either. I won't throw in Toyota ownership because a Toyota never breaks. Only needs oil changes.
Seeing that this is just a regular Z4, someone I knew joked that it was just a more expensive version of a Miata.
Now, if this were a Z4M - That'd be a different story but it's no longer an apples-to-apples comparison.
Edit: I see you are located in Sacramento. That is probably the hottest place on Earth I have ever been to, next to India. Having said that, just buy both.