Samsung Pay FTW!!! The fact that it works with older terminals is what makes it more appealing and useful, really. Android Pay just works with NFC-enabled terminals, and those aren't around much. Normal 'swipe your card' ones, however, are still pretty much everywhere and those will work with Samsung Pay.
Downside? Some banks aren't still on board with Samsung Pay (I'm looking at you, Capital One). Android Pay gets around this by using a 3rd party bank which is used for the purchase and then that bank charges your unsupported card (but you don't get charges notifications or promotional benefits from your card like bonus points for a certain category). Samsung Pay, however, doesn't have this 'workaround' so cards that aren't supported are just not supported.
Both can live in your phone, though, and you can select which one to use when paying.
Also, Samsung Pay has way more promotions going on.