I'd go for the Porsche. Never driven one but from all the reports I've read they are a dream to drive.
They are quite different cars to drive, so make sure you spend some time in both to see which you prefer. Neither is a bad car, but it will also depend on what sort of driving you do. The BMW Z4 is more conventional than the Boxster in that it is front-engined and rear-booted - and as you pointed out, there are a number of varieties so make sure you drive the exact model you are planning to buy. Be aware that larger wheels will make the BMW ride very firmly on its run-flat tyres. I used to have one on 19" wheels, and on a poor road it felt like the car had no suspension at all!
The Boxster is a better-handling device, and since neither are going to ever be considered practical, it would be my choice. With Porsche, there are a multitude of optional extras so it is rare for two cars to leave the factory the same. If you are comparing different Boxsters, make sure you know exactly what is included on each car as you can guarantee there will be differences.
As for must-have options for resale, none. Only pay extra for what you want and/or what you are going to use. Options add a lot of price to a car but add little value in 2-3 years' time. So if you want electric seats or satnav, that's great, but they won't add any real value to the car compared to what you are paying for them.
Have you considered the new Mercedes SL in basic trim, it has enough of what you really need in a car today?.
Just to add to my questions, what are the must have options for resale for these cars?
My other half, a couple of slightly bonkers army bods and a load of nutters adventures can be found at
RACE RETRAIN RECOVER