Both cards are great for control warrior, no doubt. But I would go with Alexstrasza for sure. It can be situational, but there's plenty of situations that arise, and just knowing it's in your deck changes the way you can play. It saves your ass plenty of times when used defensively, and wins games that way against out-of-steam opponents... Or allows you to just board control all game and then nuke your opponent to 15 later on, which happens A LOT.
I can't argue with the tempo that Cairne provides on turn 6, and can't imagine running the deck optimally without him... But if I had to use only one of the two cards you're pondering, Alex wins me more games I think.
Not trying to make your decision any harder, just thought I'd sound in. Looks like you have a good grasp of how to play this type of deck, so you should win plenty of games using either card!
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