Stuck between Ele and Mesmer
my vote goes to mesmer just because it is an original and more fun class to play
In both PvE and WvW, chronomancer is mostly utilized as a support role and quite good at that. Remember base mesmer is pretty worthless and you absolutely need chrono for the class to shine.
In PvE, you focus on supporting team with alacrity and quickness to maximize group dps. You also have a few other nice utilities like distortion, reflect, portal to back it up.
In WvW group fight, you are best at sharing boons with your allies. WvW group fight is always dominated by cleaving damage and ground target skills. Chronomancer also has a few good ones in this regards(gravity well, timewarp, null field). But you are kinda weak in damage dealing. You can also solo or small group roam with chronomancer. The class is quite strong in that regard.
Ele is probably a more versatile class with emphasize on damage dealing(especially in PvE group content). It is the best power dps class currently in the game but also has plenty of nice utilities.
In WvW, ele can fulfill multiple roles. Good at dealing damage, healing allies, providing area control.
Why not both?
Ele is my main for PvE, larger WvW, some roaming too.
Mesmer is my main WvW class, I bring a lot to fights, shared invuln, stealth get away, CC for druids or interrupting stomps and bursting down someone who has to die etc.
If you’re looking for absolutely 1 class that will do you good through it all and you won’t be excluded because “we have 1 already” then go ele. See that’s the thing everyone misses about chrono in PvE, they only need 1 so if you only play mesmer and they already have a chrono they will not let you play as mesmer, nor should they.