Val, in case i dont get back into town before you beat it:
The level 43 mage guild quest is the last with that guild. The rewards are the big thing. While trying not to spoil it, they will give you two options. On one hand they will blatantly tell you what you will receiver(and its pretty significant) and on the other they dont mention it at all. The reward they dont mention is ****, just some green item. I went with the item they told me about and didnt regret it.
If you still have questions, id highly recommend googling the quest when it comes time to choose, people have bitched up a storm on it.