I played a game last night and I intentionally tried two things, Not take Vader, and Take more TIE Advanced. So I took Maarek two Storm Squadron.
I thought the Storm Squadron performed pretty well, but Maarek, not so much. Its not that Maarek was bad, but he is just two points less than Vader. Spending those two points to upgrade Maarek to Vader seems to be the best two points you could spend.
Vader is sooo cool and Maarek's ability just doesn't come into play that much. You have to get a shot, that shot has to hit, you have to score a crit and your opponent has to be be out of shields.
Vader on the other hand get's to take two actions every turn (as long as he get's actions that is) Not only is an extra action better (IMO) than Maarek's crit ability, he get's to use it just about every round, where Maarek's ability get's used about once a game.
I just cannot imagine a situation where you would take Maarek instead of Vader. And I think if you were to take them both that would just be a very expensive 2 ships, especially for the Empire.