Start lvling Mes/Nec or wait for Revenant
Well since you can start Revenant in just under 26 hours from now if all goes smoothly with the launch, you may as well wait. Possibly catch up on sleep tomorrow so you can binge play as of release.
Both necro and mesmer are tons of fun though, in my opinion — it’s highly subjective and every profession has its cheerleaders and its decryers — so you could work on one of them a bit if you have the urge tomorrow. I’m partial to necromancer because reaper is so glorious, but if you can handle more complex keyboard dancing than I can, mesmer (my favorite vanilla prof) leads to chronomancer.
If nothing else you’ll repeat some events and such as you level each profession and thus get a comparison as to which you most enjoy out in the field.