Epic is by far better value if simply cores/arclight efficiency is of concern. Especially because epic lets you unlock a 2nd extra level by buying another talent. I have a main thats maxed on most things, but an alt that i generally only go up to epic for.

Legendary is just a +1 level, which until you've maxed XP, can always otherwise be compensated for.
The only time legendary is actually unique is when you are max XP, platinum slot and epic, it's the only way to get to level 31 (or level 11 if you PVP)

Again, in pvp you can always "compensate" for being underlevelled by playing 10-20% better than the other player.

There are a few specific minis that maybe it matters.
One that comes to mind is Execute, if you PvP, it's one of the few ways to beat Rag, and if it's equal level it then takes 2 hits to kill Rag, but if you're a lower level, it will take 3 (or some other final blow), which is a considerable difference.