At level 31 basically the only question u should have is what way u want things dead
Pew pew(MM tree)
Kaboom-zap-burn(Engi tree)
Dot dot dead(lethality)
At level 31 all 3 are pretty strong, altho I'd say engi has more to gain in later talents. Lethality can be a pain if things die too fast, which yes, it is a problem. MM pretty much has all its talents it needs to be sustainable at this point and is a very reliable spec to level in, and it performs exceptionally well in endgame for the amount of skill it takes, so it wouldn't be bad to learn
Basically at your level you can choose any of the 3 main specs to play and have little problems. So whichever sounds more appealing go for it! My personal choice is engi, because it is fun as hell when you get all of your AOE