If the question is more specifically leveling through normal gameplay vs. power leveling, then normal gameplay 100%.
The part that sometimes bothers me about this game is that it requires three full runs through the main story to reach level cap. Never seen that before in any game, and it has burned me out on BL2 in the past. Probably will again in the future too. I have to carefully manage my time between repetitions. Like many players I have all six characters active, at various stages of various playthroughs (Maya at 72, Axton in early UVHM, Krieg in mid-TVHM, Sal in late NVHM, Gaige in mid-NVHM, Zero in early NVHM). Now that Maya is at level cap she should be a good way to break any monotony that might set in with the other characters. That’s what I’m hoping anyway.
I’ve had many more alts with much more repetition back when I played MMOs a lot and it didn’t burn me out the same way; I think it’s because the alts were spread over 4-5 years and there was only one repetition of any given content within each character’s progression.
And the main reason I’m in no rush to try Digistruct Peak and don’t expect to ever reach OP8 is because with 18 repetitions of the main story already lined up (6 already completed), doing further resets is the last thing I want to do. I’ll take Maya as far as her level 72 gear can take her, upgrade with farmable stuff (Hammer Buster, Bee, Hornet, etc.) and pick up some Lady Fists - I’m deliberately saving that quest for exactly this purpose. And when that isn’t enough, that’s where Maya stays.