I have one of each and enjoy them both. I only play PVE and have been focusing on my Ele in an attempt to get her to lvl 80 and then I will go back and focus on my Mez more (she is currently only lvl 22).
What area will you play mostly? For PVE they can both do well, so I'd say play them both and see what's more fun. For PVP, they both have their sport, though tank ele is in the current meta more; but I hope you like fighting mele which you'll have to, and taht doesn't sound like what you want. If you'll be doing WvW and your server isn't full of dumb people, roll a mesmer, they are VERY valuable there, even with some recent rnerfs to their portals. (and more to come likely)