Help with profession
go to the player helping player section. There are several similar posts and some very good advice.
what would your play style be?
thief = stealth
mesmer = illusions
Neither is better, but both are fun. Both are also professions that can be challenging at the beginning, but come to their own later.
Builds aren’t very important until higher levels, so no need to worry about that yet. First get to know your character or characters and see what type of play feels the most fun to you. Then find the builds that compliment that.
My recommendation is that you try them both. And by trying I mean actually getting them to at least level 20, preferrably 30. Then you pretty much know what they are made of. Pick different races or at least different stories at the start so you have a fresh game for each.
Once you are in game, feel free to whisper me, if you have a ‘silly little question’. Or even if you just want to chat.
GW2 is not so much about which is “better” in the traditional MMO sense, since there are so many factors influencing the usefulness and feasibility of your character and there is little to choose from in a sense of one profession being overpowered to another that it mostly comes down to play-style.