
Mesmers come into their own at higher skill levels, and so do Necros, but not to the extent of mesmers.
What I would do is up until lvl 10, focus on the necro abilities, and then later start working some really good mesmer tricks, since many lvl 10 and up mobs use hexes and mesmers have all the counters.
A few mesmer abilities makes the first few levels real easy... like empathy (which redirects damage back to the source)... great way to kill of those tough elementals by watching them kill themselves.
As for many necro abilities, that I am not an expert on, but my Monk/Mesmer (now at lvl 17) can REALLY be annoying with the mesmer spells I use

Jana