Underpowered: I'm not really sure, I just lump some heroes together in the "not useful been bunch" group in my mind, so no one of them really stands out. I have been pretty disappointed by Mystic Punk in Trials.
Overpowered: Grizzled Hunter. He just shoots all of my heroes down in the Arena, even if he's 5 levels lower and it's all my back-line heroes versus him. I don't know how it works, but his skill of getting stronger and/or shooting heroes should be finetuned.
Heroes I'm trying to get/evolve in Elite Campaign:
In order, Hex Witch, Storm Wizard, Grizzled Hunter.
Currently spending stamina doesn't seem to be as worthwhile as getting all the new equipment for my remaining heroes, but under normal circumstances, yes!
For the Arena I feel like I just need to own the 5 right heroes for a balanced team (I know I should use different formations, but I'm not yet at that level yet with enough heroes). For the guild war it's the same, with the top 10 right heroes.
Since experience is partly buyable with gold (and I buy it) I'd say gold. Also since I'm currently focussing on getting my skills to the max for more heroes, I'm more often short in gold. Experience does clearly become more useful as you progress, so first it's gold, later on it becomes experience. But I think that's a fun 'twist' to take into account.