Purchase advice GTS 2018 vs Gts 2015

Hi together, would like to buy me a GTS 911 coupé or convertible. My questions about this are: Which model is later higher in value ? the current 2018 or the pre-model 2015 ? Which engine is more fun-wise to drive and above all louder? Suction cup with the 4-pipe system or the current with the 2 pipes and the 2 turbos? Optically I like them both, of course, as last one of the two models is deeper than the other? have an underground garage, but I come with a C Arrera S there loosely in without putting on, as the driveway is really not steep to luck. Thanks already for the help and a nice evening to continue. Lg Marcel