I've got a mesmer to mid-twenties so far. Staff and Sword/Pistol seem to be a very popular weapon choice, but I'm questioning it. With other professions, I find myself picking two different weapons for versatility. On Guardian, for example, greatsword to mow down groups of mobs, then scepter/shield for when things get heated and I need to get some range and defense going (overaggro, or champion fights).
On the face of things, Staff and Sword/Pistol seem to make sense for the same reason. Conditions/range on one weapon, melee/high damage on the other. In practice, though, Mesmer seems built to do a lot of weapon switching. You only get one phantasm on a given weapon set, so if you are going with a phantasm build and relying on them to do a lot of damage, you're going to be swapping back and forth to keep two up. If you are going clone spam, then you'll need to do weapon swapping so that you can throw out three illusions, shatter, then swap to get fresh cooldowns for three more illusions.
Because of that, it seems like the two weapons sets need to work hand in hand, rather than having two that shine in very different circumstances.
I don't know where that leaves me with my ideal weapon set, but it might be something more like staff and scepter/something. Still working that out (been playing my Guardian a lot since I decided to rethink my Mesmer weapons).