Depending on the build date, 2016.5 S is still missing several things from a newer S. Below is a list of features I collected when we were shopping (specifically for an S):
1) Used RWD Model S will have a bigger frunk (front trunk), big enough to swallow a stroller. The frunk has shrunk over time due to AWD becoming standard but also due to bigger air filter for Bio-Defense option.
2) Premium Rear Seat design began July 22, 2017. There were four generations of seat designs and recent years had a mismatch between front and rear seats.
https://youtu.be/Ih7WqPFAx00
3) All-glass roof was an option since 2017 and became standard Nov 2018.
4) Rear cupholders (and rear USB ports) started 06/2016, so get a July 2016 car to be sure you have it (if that's important to you).
5) MCU2 introduced March 2018. The CPU update makes the main screen interface quicker and more responsive. ***UPDATE: Since V10 Software Update in Sept 2019, only MCU2 cars have the processing power to handle Theater Mode (Netflix, Hulu, Youtube when parked). ****
6) Raven update provided 10% more efficiency/range and new smoother air suspension, started the last week of April 2019.
7) Facelift cars appeared mid 2016 (new front instead of black nosecone).
8) Sentry Mode is available on all Model 3 (of any year), but only on Model S with a build date of September 2017 or later because you need at least Autopilot 2.5 for Sentry Mode to work. If you have Autopilot 2.0 then Sentry Mode “works” but it doesn’t record to a USB drive, instead it records to “Tesla cloud servers” but nobody has access to the videos!
As much as I love our S,
between your two choices I would strongly lean towards the 3 for the full warranty, longer range, newer hardware with Sentry Mode, and faster Supercharging. If you need more space, then maybe save up for a Y or newer S.