I also was undecided. The 6th gen SS was improved in all aspects: lighter, better handling, same Stingray engine and A8 transmission.
What help me decide was that the 5th. Gen has a usable rear seat (no adult or big kid could fit on the new one) and the 5th. Gen ZL1 has a more muscle car type look, a stronger 580 hp engine, mine came with stock Recaros and its drivetrain is bullet proof. I did not trust the new A8 transmission after reading the bad experiences on the Stingray and GM pick ups, mine was a '14 with very low mile just 1100 at a very atractive price.
You will not go wrong on any choice that you take 5th. Gen ZL1 or 6th. Gen SS both are great cars.
