Coming back can't decide
Most people would agree that Mesmer is rather boring to level until level 40 or so. I personally didn’t since he was my first character so I had no comparison.
If you read around the forums you would probably know that Mesmer is pretty top tier in end game. We are great in solo PvE, events, dungeons, all sort of PvP.
I never played Necro so I’m not going to try and give you potentially false information.
As said above, be prepared for a bit of a slog till you get to the higher levels with a Mesmer. Also, if you don’t like switching weapons frequently, don’t play a Mesmer. Later Mesmer can be spectacular, but mainly it’s a “busy” playstyle, so if you don’t like “busy” you won’t like it. In esssence, it’s a CC/damage class.
If you like pets, play a Necro rather than a Mesmer. The Mesmer “pets” (illusions) aren’t really pets, but more like distractions combined with grenades, including CC grenades.
Mesmer playstyle is much more action packed, but necro with a staff+well AoE build can be very comfortable. My lvl 80 necro with such a build can sit in one place and take on multiple enemies (pve) while throwing marks under their feet and popping up wells. It can be very tanky. If you’d rather play to relax, i’d say go necro. If you want a bit more action and difficulty, go mesmer