Get a FreeSync one now, they're not much more expensive than a regular one and you will benefit fully from it with the RX 480.
If you ever wish to change your card in the future to a NVIDIA VGA the FreeSync will go to waste tho, but i believe that until there you will have already experienced everything it could have delivered.
Btw, G-Sync monitor don't work with AMD Video Cards, so for now that option is irrelevant and they are dead expensive too, i'd recommend you grab only you wish to upgrade to 1440p 144Hz gameplay.