by North_Wolf
Yig has a very short doom track, so unless you play an amazing game then you are not likely to win by sealing gates before the doom track fills up.I would normally advise newbies to use Az but given the time constraints and the number of players I think that Yig will suit your needs better (the shorter doom track means much fewer game rounds before it ends).
Just explain to the players that beating the game with Yig is very very very hard, but the final battle is a chance at redemption. :) The grognards around here consider beating an AO in a final battle to be a draw but you can celebrate having prevented the end of the world.
Just spending the game preparing for the final battle would make it pretty dull in my opinion so you may want to encourage them to accomplish the impossible (beating Yig before he comes out). This makes for a great challenge, alot of feelings of doom and is very Lovecraftian.