A year from now Android will get cool new features and Samsung will take forever to update, and will of course update their 2016 flagship models first which would leave me feeling frustrated. A year after that Android will again update and at that point your phone will be way down on Samsung's update radar, in effect almost forcing you to buy a new device to get the new version of Android in a timely manner. I'd recommend a 5X or 6p also.
I feel like a year from now, Android will have force touch, and old devices won't support it anyways.
But you get a lot of stuff
upfront with the S6 Edge:
-Higher Res AMOLED screen
-Smaller package than 5X, more usable screen space with HW buttons.
-Better SOC
-More RAM
-Edge functionality. It's a nice feature, IMO
-Better Build Quality
-Better Camera
-Micro USB, so can use current cables
-Wireless charging
No Android update will add those features to 5X.