The splinters and Gs are one time only per account. All other rewards you can get on other difficulties, like hp/sp increases or power points. So after you beat the story once, there's no real reason to do the whole thing again. I mostly just do legendary quests, terminals and x-def.
If you enjoy daredevil, get him up to 60 first before starting on another character. For leveling one character up to 60, you get either 25 or 30% hero synergy bonus xp. You can look up that bonus xp at the bottom of your hero stats, and see how it works exactly.
That's good to know, I will just keep doing other stuff then. Though I guess I should ask, I noticed there are different difficulties, do those effect stuff outside of the story missions? I haven't unlocked the last one yet, obviously.