^ sounds like the Z4MC in terms of DDing with the stiffness and long trips. I do it all the time, but my father dislikes my Z4MC for that sort of thing (too stiff for him and tight of a cockpit).
Also sounds like the Z4MC in terms limited modding since it was a more limitedly produced BMW.
The main difference make comparing Z4MC to Cayman S (performance aside) is probably the maintenance and DIY abilities. I'd rather work on the Z4M with all the space in the world under the hood vice dropping the engine out of a Cayman. That said a Cayman is still a great car - that was just always a personal 'worry' of mine.
OP in terms of pricing look what is out there and figure out if that's what the market says the cars are going for. The car is worth what someone will pay for it. If that's a desirable color combo... someone might be willing to pay more for it. Comparing a Cayman to a TT I presume? Not sure what else would be analogs of each from Porsche to Audi.
IB ZHP E92 + 911 carrera project car
E86 Z4M - Sold
E36 M3 - Sold