Both are great airplanes. The Kaos in all it's forms is fantastic. They are good sport planes and ultra easy to land.
The Dirty Birdy was designed later and has several advances. The tail is a little longer so it flies smoother. The wing is thinned a little and it is fast. The stab is built up too. This plane will slow down nicely for landing but it is lower in drag and you have to throttle back and start your descent earlier, no big deal. But the real difference is in knife edge handling. The Dirty Birdy has more side area and is much better in knife edge and point rolls. To top all that out, the Dirty Birdy is one fine looking airplane.
So for basic sport flying, a Kaos is great. For more advanced aerobatics, the Dirty Birdy is much more capable.
If you are interested, I have written updated instructions for the Dirty Birdy. Look here
http://www.rcuniverse.com/forum/m_9520759/tm.htm
Dave