Both are good choices - I'd describe the difference between them as Mystere being Cirque for beginners. It's less "razzle dazzle" than the newer ones, but it's much more of a pure Cirque for the acrobatics contained in it. It's a good one to see if you're not sure if you will like a Cirque show since tickets can usually be found for a very cheap price. It's the longest running Cirque in Vegas and they offer lots of discounts for it.
Ka, being a later Cirque production, is much flashier than Mystere - lots more effects, faster action, etc. It's a really interesting contrast to what you see in Mystere.
Both are well worth the money if Cirque shows are something you enjoy!Like treegirl mentioned, try to sit midway back in any Cirque show - the high priced front row tickets are not the best seats for seeing the aerial work that goes on, or for having a good view of action that can take place all over the theater.
