The primary issue is the condition of the battery and motor. The 2014 Turbo S has a 504 Wh (14 Ah) battery while the 2015/16 Turbo has a 468 Wh (13 Ah) battery. Neither has the the bluetooth connection. The Turbo S has a 250W motor (same as 2016 Turbo X). Either should be able to be upgraded using the newer 691 Wh bluetooth capable battery. This is a substantial issue wrt. warranty, however. If the base Turbo battery fails, it will be covered under warranty. When mine did, I was able to upgrade to the SBC-05 (691 Wh bluetooth) battery for the price difference ($200). If you can get someone to run the LBS diagnostic on the 2014 Turbo S to check battery life indication, it would be a good idea.
The frames are (for all intents) identical. The 2014 Turbo S has somewhat upgraded mechanicals with SRAM X0 instead of X7, 11-36 cassette instead of 11-32, and Formula R1 instead of C1. However, I would maintain that functionally, with an upgrade to a 10-speed 11-36 SRAM cassette and some organic brake pads like Kool-Stop, the base Turbo should be just as good as the Turbo S. The 2016 Turbo S, OTOH, has MUCH better mechanicals than the 2014 Turbo S with Shimano brakes and Deore XT 11 speed mech. I did the Shimano XT 11-speed mod to my base Turbo as well.
Were I in your shoes, given that I already have a base Turbo with the uprated battery and Shimano mech, I would get the 2014 Turbo S and then swap components I already have (shifters, battery, fenders, etc.) over to the Turbo S as well as install new brake pads. I would clean up and put the SRAM and older battery on my base Turbo and restore it it to near factory condition. I would have my dealer do a diagnostic on it and provide the report when I resell the bike. In your situation, it all depends on how much local support you expect from your LBS and what confidence you have in the battery.