Low Damage - Mid Level
It depends…if you mean burst, then the thief will always be a bit stronger. However, in terms of overall dps, the elementalist will do just as well. In terms of AoE, the elementalist wins easily.
As for when that will happen, it takes a bit more practice and build refining to deal massive damage with eles than it does with other professions. Some of the better traits won’t be available until later in the game, and a good part comes from your rune and sigil selection as well (which you’ll likely leave until level 80). My advice would be to read some guys to get some insight on how to make an ele work, and try it out a bit while levelling.
Stick with Thief. Currently playing Ele is like playing the game on ultra-hardmode with no reward, just a lot of frustration.
It’s totally possible to play an Ele quite well after lots of practice & reading/watching, but if you apply the same amount of dedication to your Thief you will have a better experience, especially at lvl 80.
Have fun