If you have not completed the Mage's Guild quests, this will be a spoiler. If you are good with that, then read on.
So you have your choice of either getting a book worth 2 whole skill points (maybe 30 minutes work to just go find some skyshards instead) or saving Valaste (she says thank you really, really sincerely).
Seriously? If you want to create a moral dilemma you are going to have to do a lot better than that. So what should be in the Folium Discognitum to really tempt players, and what should be the alternative?
I say the Folium needs to contain something valuable and unique, like a whole skill tree you can't get anywhere else. Or knowledge of how to create a powerful potion.
So, what is for offer on the other side? Perhaps having Valaste as your friend. Friend as in, she will go on a limited number of quests with you. Perhaps she should have already gone with you on a quest or two and have said that she would like to keep accompanying you. Or maybe she will marry you with similar benefits to those you get in Skyrim, in which case the romance should already be in progress by the time you have to decide. Then if you save her she could make a nice speech about how books are not the only thing she loves.
These are just a few possibilities so don't get too hung up on them. The point both sides should give you something really tempting and UNIQUE, not anything you can buy in the crown store or find in other quests.
So what do you think Sheogorath should offer, and what should you get if you turn him down?
Seize the Dungeon Master