First of all NHL 15 was NHL 15. There was no 'legacy'. Second, this is a heavily personal opinion weighted question but I think NHL Legacy is fantastic. Would I like the 3-on-3 OT? Sure, but it's not going to ruin my game. I need the current schedule and a handful of newly made players. That matters to me. The game is as great as it was last year to me and perhaps plays a bit better but I have no quantifiable way of checking that because I haven't read anything that says they did anything to Legacy.
As far as the nextgen version goes, I've seen lots and played it and honestly, there is something in sports games when they go to new generations that never ceases to amaze me; the overdone graphical elements in place of finished games. Almost to remind you by beating you over the head that the game looks better because of a better GPU but almost as well to distract you from the fact that the game's launched unfinished. Look, if you like the latest in graphics (and even that is questionable) then next gen matters to you.
When I play hockey, baseball, soccer games, I'm watching the ball and my teammates. So if there's a guy walking up the aisle holding a realistic bag of popcorn or if the stadium mascot runs onto the ice at the end of the game couldn't possibly be less relevant to me. You know why I still play MVP NCAA Baseball? Because the physics and the gameplay are so tight it doesn't matter that it has PS2 graphics.
Anyway, just some thoughts.