I've never wanted to train a Niss or Fred cause a damage dealer pet needs much more IM investment to be an useful pet. =P

But, since Fred is a fire he gets more MATK bonus from his element plus till RB of pets, he got a single magic attack which means he can deal more damage.
But there are different situations stated over here and official forums from experienced players like,

Niss is a water and in future most of the bosses are fire, etc.. so this means fred won't be able to survive till he gets a cast, and ... in the long run Niss would be much more useful (Can't tell much about this)