I'm a dueller and thus try to get as much SE as I can for the XP I get, my question is:
Which is the best skill for a dueller: "energy harvest" or "disadvanced development"

From what I can see, if I get get 100 "spectral manipulation" and 50 "disadvanced development" that would be better SE/XP then 150 "energy harvest" But at the moment I do not hunt, unless I need to to complete a quest, so I'm also wondering if it's worth changing that at all, from an SE/XP perspective.

It depends on your duel gains.
disadvanced devellopment halves the XP you get for hunts.
spectral manipulation + squeezing + trapping can roughly tripple the gains.

So if you 6 times the no-skill SE/XP is better than what you get on duels, it would be worth it.

Personally I think you're missing out on a lot of fun by not hunting at all and staying low level. Every few levels extra features get added, so levelling up is well worth it. With max hunting skills and disadvanced development, you can be sure to stay on top of your level while going up.

Energy harvest only works if you'd get little SE on your attack otherwise. I don't think it's worth it for a dueller who often hits people who give SE even without that skill. And if you get lots of 0 SE duels, the SE/XP ratio from hunts with max skills is probably better than from duelling.

Energy harvest is good for hunters at high levels who randomly hit people for XP, they often hit for 0 SE, gaining a few SE each hit can add up to nice numbers, especially since the gains are level based.