This is a generic question that I hope can answer to your content as you did not give specifics.
Either class can solo will at 70. If you're wanting to solo raid content well ... that ain't going to happen with a bot unless you so far outgear the content that I wonder what you would be doing in the zone by yourself

As far as being useful for guilds you would need to contact the different guilds and ask. We have a guild forum and the more active guilds often update their recruiting roster as each guild has their own needs.
Of the two classes you listed Necromancers are "safer" dps, being as they have a number of lifetap-like spells to keep them standing for the duration. That being said, a wizard with ldon spells blows everything else out of the water (and blows the water up with them). With a mana clicky to replenish yourself and some ldon points for spells you have instant short duration dps that can't be matched for a comparable effort.