K2 makes good stuff and I was considering the Believer and Darkstar until a great deal came along for the Rome Agent I bought. If you want a freestyle board from K2 I would go with the WWW if you want PURE freestyle and the Believer/Darkstar if you want all mountain.
Once again, sedition with the good advice, buy a setup for mountain and another for freestyle. I just replaced my very first, and 8 year old gear with a new all-mountain setup. Eventually down the road I'll buy a freestyle/park board to use when I'm not snowboarding with my wife
. Also like Sierracrew mentioned, weight and height should both be considered when sizing a board. Most manufacturers have sizing guides for the boards and if you're looking for a freestyle only board you'll want to be on the upper limit of the weight for the board so you can use your weight to manipulate the board easier.