I’ve owned a Boxster, Boxster S, Z4 3.0 SE, and a Z4 M Coupe.
If you want to mince around in comfort with something that sounds great and handles superbly, a Boxster is the car for you, if you want to add speed to that, then Boxster/Cayman S is the way to go. If you want reliability, a great sound a much better interior and a fully electric roof, then get a Z4, but ditch the RFTs straight away. If you want an out and out sports car the Z4m is unbeatable for the price. The noise, the handling, the speed…it’s awesome.
Servicing for the standard Z4 is far cheaper than the Boxster/Cayman and both BMWs are immeasurably more reliable. I wouldn’t have a 986/987 Boxster/Cayman without a full manufacturers warranty. Stories of engine failures are rife, and mine needed all the coil packs replaced at 30k miles…not cheap!
Z4 is a totally different animal to a Z3.