Deciding between Mesmer and Elementalist
Mesmer has a lot of support options when buffs are concerned, especially if you get into Heart of Thorns content and get the Chronomancer. Once you get the playing style down, you’ll get quite a few friends (or if you’re in PvP/WvW, also quite a few raging enemies).
Elementalist, on the other hand, has a more flexible level of utility. Sacrificing built-in weapon swap for attunements (which actually counts as “swapping weapons”) you can pick what is best for the situation: water for keeping allies up and slowing enemies, lightning for decent crowd control, fire for burns and straight damage, and earth for bleeds/immobilise (and depending on your weapon set, cripples and weakness on foes). Potential ramps up when you get the Tempest, however I haven’t really seen enough Tempest ele’s to get a solid wager.
Since you’re working on support with some DPS contribution, I would definitely suggest you try Mesmer first and see if it’s right for you.
Part of the [PORT] mystical tour – “Marilee Mangletooth.” What main?
Something something Autechre fanatic.
Having been a Disc Priest main myself for quite some time I would say neither. The GW2 equivalent of how I remember Disc Priest play style (mostly spamming PW: Shield in my day (stopped playing shortly after Cata)) would be to put out the most mitigation in the form of Protection, Aegis, Blocks, Reflects, etc. possible with some healing on the side. That would lead me to suggest Guardian with Mace+Shield and Hammer/Staff.
Mesmer and its elite spec chronomancer are all about support. Their personal damage is fairly lackluster in PvE most of the time, but their support is almost unparalleled due to their massive access to quickness and their unique mechanic alacrity (reduced CDs), as well as other fun tricks like distortion share (giving the party 1 sec immunity to all incoming damage). However outside of extremely niche scenarios (currently just the raid boss Matthias) no one will ever want you for DPS.
Elementalist is a damage dealer. Incredible DPS output at the cost of being squishy. They can also play excellent support and as a healer if needed but there are better classes for that right now.