Both Ele and Mesmer are good in roaming, small party WvW.
In Zerg v Zerg, I wasn't very happy with my Mesmer. They have some great utilities, like portal, and into the void for pulling people off walls, reflect bubbles, but damage being tied to clones and phantasms, which die extremely quickly in zerg v zerg, makes trying to do damage very strange.
Ele, on the other hand, can dish out AoE damage while also having CC and the ability to support with a variety of combo fields.
I know plenty of people play Mesmer in WvW and do amazing things, but the above has been my experience. I expect that Mesmer really shine best in a coordinated group where their utility can be used to best effect. Ele seems to be able to shine in both coordinated groups and general WvW chaos.