I think I can give you some good advice for that, since I play both and both are lvl 80 full exotic already.
I mainly use my elementalist for dungeons and events, eventhough I used to spvp and WvW a lot as elementalist. Elementalist is pretty usefull in teams, however, it's not a profession that's made to kill. This, however, does not mean you're useless in 1vs1. As D/D and staff you can find your ways around to win a lot 1vs1's, as long as you know the tricks.
I use my mesmer now only for WvW. I help groups out getting into keeps, roam with it and win 1vs2 or sometimes even 1vs3, just because you can create so much confusion that you basicly kill people, without them even noticing who killed them. I love the whole confusion part of mesmer. It's like you're playing with someone else's mind.
However, what I don't like about mesmer is the PvE part. You have to be pretty supportive if you want to be a bit usefull for your team.
So kinda to sum things up:
Want support and PvE: Elementalist
Want to roam alone of with a few people in wvw and cause confusion: Mesmer.