Deciding between Mesmer and Elementalist...
Well, this being a RPG, i will just touch on the more important difference between Elementalist and Mesmer – how they kill mobs.
Mesmer – main mod of attack is to create clones of himself (up to 3), and summon another special clone to help the Mesmer kill mob. While the mobs are concentrating on fighting the clones, the Mesmer himself can continue to dish out damage on the mob. Also, Mesmer comes with a few class skill that utilise summoned clones for further DPS – he can choose to Shatter his clones for various effects – extra damage, conditions such as Confusion, etc.
This is basically how Mesmer kills. Plus there are some unique utility skills like create a Portal for party members, teleporting, etc.
Elementalist, on the other hand, kills mobs using their elemental spells mainly. These can be in the form of AOE spells such as Fireball, or single target spells. Each element (Fire, Water, Lightning, Earth) has different kind of attacks, and Elementalist will switch between them to maximize DPS and to suit the circumstances. This is your typical mage-type class.
In my opinion (I have played Elementalist to level 65, and Mesmer to level 61), Elementalist would probably suit you. Elementalist is a lot more demanding in situation awareness, requiring you to switch elements so that you can better survivie and kills better. It is IMO more complicated, on the edge, but ultimately satisfying. Mesmer has better survivability (due to clones), but might feel a bit boring once you figure out the best combo you like for DPS.
I found Ele to do more damage, but is more difficult to play.
Mes does less (not much less) damage but is way more fun to play.
Have you thought about Necro?
ele: very weak, high damage. To succefully play you need to dash out of danger all the time, making it challenging to play for those not used to that. You kill quickly and efficient. This makes the fights very dynamic but short.
Mesmer. Due to the use of clones you don’t have to dash away all the time. besides that you can take more damage anyway. so it is easier to stay alive. You deal however lesser damage making the fights longer. The amount of clones of you running araound can be disorientating, but when used to that you can relax annd enjoy.
As for weapons. The most used weapon for ele is a dagger (either scepter/dagger or dagger/dagger. The dagger however is a casterweapon and works as a scepter with a different skillset anyway. For the mesmer this goes for the greatsword. The greatsword is very popular among mesmers. but it is basicly used as a staff (but different set of skills).
Arise, opressed of Tyria!