Technically for survivability 55 monk is better on the one hand but on the other hand 55 necro will deal more damage.
I have both and also a 55 mesmer ( got bored one day and fancied a laugh ) but i use them in different places - 55 monk in gok/bergen/fahranur ( anywhere with undead ) but 55 necro i use sometimes for solo boss and a few other places.
55 monk with sv and necrosis if you have nightfall is a gd mixture.
There is a thread somewhere to do with a necro farming guide and that covers 55 necro and that may help with where/which to farm with.
Some will say why have 1 when you can have both
