Mesmer gets a lot more fun as you level. Especially once you get clone on dodge...they become kinda OP.
Key to playing a mesmer is really clone management and shatters imo. I personally use a GS and S/P on swap for general PvE.
As for thief, 100% crit on stealth is a terrible trait for PvE. Thieves will constantly be getting nerfed in PvE as a side effect from nerfs in PvP, because all people do is complain about the class unnecessarily.
Also, thief's squishyness does not go away at higher levels, but the same can't be said for mesmers as they actually become quite good at survival if played right.