First you need to realise that magic generally is not a battle winner for the Orcs and Goblins Army.
There are a couple of real killer spells in each list though.. How ever much focus you do place on spells you definitely need to take a shaman into battle against most enemies if only for the dispel scrolls to help nullify their magic.
Gork magic is probably slighlty weaker than Mork magic although casting points are a little bit higher. There is on exceptional spell Mork Wants Ya! Which can take out enemy characters and beasts if you get lucky.
Gork magic is overall a lot cheaper simply because Goblin shamans are cheaper that Orc shamans.
I don't recommend taking any Orc shamans at all unless you really want the WAAAGH! Spell to propel your troops more quickly into combat.
