I have an oppurtunity to get a Promo Emrakul or a pack Ulamog, which would be the wise choice? I know Emrakul is used in Legacy, but aren't prerelease promos always valued less?

I personally would go for the Ulamog. Promo Spaghetti Monster looks meh imo and Ulamog sees play in edh. Although I wouldn't be surprised if Emrakul ends up having a higher price ceiling. Promo Emrakul is about $2 less on TCG atm.