So I've played Warframe for 400+ hours in the past, haven't played nearly as much Destiny but played a good number of hours still.
It depends how you look at the longevity of each game. For me Warframe doesn't have the grab to keep me going. I wanted to get a certain Prime frame, and it was fun getting to that (I played with a bunch of friends), but once I got what I wanted I realized there's literally nothing else to do except get more frames and weapons. Some people are okay with that, but I didn't feel like I was really progressing or working towards anything, the game is just "get more stuff". In that sense Destiny appeals to me more the same way Path of Exile does where you actually build on a character, and have an end game goal that gives it purpose.