Both are pretty similar really. Far as being fun I can't say one is better then the other. I do know necro spells generally cost more because there seems to be more necros then summoners (at least on unrest). It is always fun having a evil race i a good city and vice versa though. Something you should also know is the betrayal changed awhile back. Before you had to do a handful of things then kill 500 gnolls, handful of named gnolls, then a few other things. Now you get to do alot of VERY and I do mean VERY repetative quests over and over again. Alot of people like it, others like myself don't. In any case something to consider when you go to betray.
I played a summoner shortly after the game first came out to 50 (was as far as you could go then). The only real difference I have seen between summoner and necro is the way you heal your pet (at least pre 50). A necro has to give there life to their pet as where the summoner don't. Necros at lvl 56 get a really neat undead charm, not sure if summoners get a charm or not.
All I can say is look at the spells of each class and decide.
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Hi