I levelled as an engineer because it has the best AOE out of all the specs and a majority of the time your fighting multiple mobs, plus once you get cluster bomb with a full upgrade and EMP you rarely run out of energy. Plus you can REALLY annoy people in voidstar by placing a plasma probe on the door and running the hell away lol If I had to choose between lethality and marks I would choose marks, as it is the best burst damage spec there and you can 2-3 shot most things. But if you want to do some interim pvp then this would be a better choice. My opinion is a bit biased due to the fact that I havnt tried lethality but from what I have seen/heard its not a very good spec. Hope this helps