Festival is like almost-strictly-better-than-Silver, hence it costs $5. It's not as good as Village for drawing the deck, but it can be situationally stronger with draw-to-x and helps you skip buying treasure payload. Festival does suck when it's your only splitter and you really need to line it up with your draw in your opening hand. Without support, that is.
Edit: Okay, to help make it clear what I am trying to say, think of how you feel when you get an opening hand like Festival-Festival-Festival-Moneylender-Estate.
You know, Grand Market usually dominates in games with trashing where you don't really need much Laboratory-style stuff to play every GM in your deck. Not every game has super fast trashing though, and Lab helps you get to your trasher and maybe your attack faster than GM. Later you can get the GM and steamroll.
I would say GM and Lab accomplish different things, but I rank GM higher overall because it covers payload acceleration dramatically in a way Lab does not feel like it does for deck reliability. The +Buy with the GM gives so much pile control. Every GM play is another potential Estate you can buy to help close out the game on 3-pile.