Quoted:
Both the 2.7L flat 6 and 3.4L flat6 have proven to be incredibly reliable.  

The pick is the 3.4L so 981 Cayman S - its a sweet engine, raucous sounding, revs to around 7,600.  As long as you do regular servicing they seem to run forever.  

Another point to note, the 981 and all modern Porsches are mainly aluminium with aluminium underpinnings which means they last well in all environments.  The factory rust warranty is 10 years.  

If you like the driving experience, fast corners and NA engines its really hard to beat Porsche. Keep in mind the following generation (718) went to turbocharged 2L and 2.5L 4 bangers and kept the body from the 981 (with minor cosmetic differences) for good reason.  Great looking and very genuine sports car.

As a point of reference, and with out going into it, I have quite a few very nice pieces of sheet metal.........one of them is a 981 GTS (with the 3.4L flat 6).  In other words, I followed my own advice

In terms of maintenance issues with Porsche sports cars - I have owned them for 20plus years and in that entire ownership history I have had only one failure, that failure was a water pump on a 991.2 GTS which was replaced free of charge (defective batch of water pumps that Porsche were aware of.

Another point to note, I read a statistic somewhere that 70% of Porsche sports cars ever built are still on the road.