Ive never played Valygar, but Keldorn is VERY good. His dispell ability will save you lotsa times, and he can wield the Carsomyr, which is a big point in favour of him. He can wear all types of armor unlike Valygar (his own armor is very good).
On the other hand, Ive heard that Valygar is a beast if you equip him with the Celestial Fury and his own sword. I guess you should give him the shadow dragon armor and one pair of boots of speed (if ya arent using them, and i doubt it).
So, if you are a paladin, take Valygar.
If ya have proficiency with katanas, take Keldorn.
Other wise, take Keldorn too i would say.
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"Let my spells do the dirty work" Ertai, High Sorceror of the Holy Flame