This came into my youtube feed yesterday. Cut & paste Harris's comments from 3:10 about M2 CS vs GT4 to GTS vs GT4.
What kind of driving experience do you want? They aren't close to the same. And for now it's a 6MT vs DCT question as well.
On current values, warranty aside, you're 110k+ for a GT4 with buckets. Somewhere upper 70's to 90k for a good example GTS, but if regular track, some mods are required. GT4 is ready to go from the factory for the majority of drivers.
Do you enjoy tinkering? Wheels on and off, adjusting 3 ways. GTS isn't given enough credit for that.
Never driven a GT4 of any gen, but have many sessions nose to tail with the 981 in my modded E90. Pretty sweet car, but never came around to "needing" one.
Early days, but sensing a bit of a lukewarm reception from Porschephiles on the 718 GT4. Might be much better value a couple of years from now and CPO.