You're not choosing between Me/N and N/Me, you're choosing between a necro and a mesmer.
Secondary profession isn't anything permanent, once you get through the initial stages of the game and get to lvl 20 (yes, 20 is the start. Everything before 20 is a tutorial) you can change your secondary at will, so it shouldn't be a part of your character's "identity". Usually, people just use a couple skills from their secondary to suppliment their primary specialization.
With that in mind, I'd recommend a necro for PvE because in PvE they are ridiculously overpowered and anet is content on leaving it that way because necs aren't breaking PvP (whereas other classes suffer nerfs periodically). How are they overpowered?
- Energy returns from soul reaping are directly proportional to your rate of killing stuff and your "rate of killing stuff" increases if you have unlimited energy. Just ridiculously overload your group (heroes, if you prefer) with damage.
- Spiteful Spirit (SS) single-handedly outclasses any other AoE damage spell in the game. Yes, Sandstorm can deal more raw damage, but Sandstorm scatters enemies and ends up dealing less. Searing Flames is more effective in multiples (3-4 SF eles in a group) but a single SS nec will outdamage a single SF ele and will still have time to do /point and /laugh.
- Minion Masters (MM) necros do a lot of damage and minions are often more effective at "tanking" than a dedicated warrior tank because they create a wall between the monsters and your party.
I'm not knocking mesmers in any way... in fact, my favourite character atm is a mesmer, but a mesmer can only really shine in PvP. Their specialization is messing with opponents and in PvE you don't really need to mess with anything... you can just kill it in 30 seconds and move on. Why use a variety of precision tools when hammer works just fine for everything?