There are minor differences in abilities between servants, though they may not even be noticeable (supposedly one necromancer concentrates more on melee resistance and the other on magic, but effectively this doesn't have a huge impact on your creature, with a couple people saying they didn't notice any change in stats or effectiveness). I didn't really like Igor's voice or Allan's personality...
The main difference between servants, as D1ablo said, are the platform upgrade quests and rewards (see
Sentinel Island Servant picks....).