Been in this situation, driven both extensively.
Cayman S
Though I purchased a 911
Do a search on this forum for my write ups on both cars as I went into quite some detail.
Basically TT RS is hugely quick and very tunable for very little money to make it R8 V10 quick for circa £2500-£3000 making it an epic straight line car and great for 1/4 mile, it handles well and is fun to drive, but its not a true sports car.
The Cayman S is not much slower in a straight, but is a whole lot more fun, grin inducing to drive, its a scalpel in comparison to the TT RS hammer approach.
If it was between the 2, the Cayman S for me, its a far far more rewarding car to drive on a spirited country lane blast, will probably hold its value better too. If you mainly sit on motorways then get the TT RS as it will be better suited to motorway use but if you ever just go out for random drives, hunting down great roads, well the Cayman S is in a different league when it comes to rewarding the driver. The TT RS is a car that impresses you but would soon becoming boring and lack involvment, wheras the Cayman S is a car you won't bore of easily. Buy a Cayman S and for 50k, either get yourself a Cayman R or a Cayman Black Edition, both stunning cars. Or be sensible and buy a 3yr ish old 911 from an OPC with 2yr warranty and stick a private plate on it, because the 911, well just do it!