I think based on the flex ratings on GNU's site, I'm second guessing either.
The K2's I worry about play in the binding, and seem like they have very rigid bases.
The GNU bindings seem the same - the Parks I was looking at have an aluminum base, and a stiffer rating. Strap comfort is important to me as well, which is an unknown.
I have been checking out Burton's 2013 line, and the bindings that seem best suited for this kind of riding seem to actually be the Custom Restricted (re:flex), over the missions - or the Cartels I used to own - and they actually cost a tiny bit less. The restricted's have the upgrade of the asymmetrical strap over the basic customs.
On that note - a buddy of mine who lives in California recommended Flux bindings - I don't believe anyone even sells them locally so that throws a wrench in the process a bit - but I'm interested if anyone has any comments about Flux bindings that would compare to a Burton Custom (good flex, comfortable, secure, with bulletproof rachet hardware)? He didn't mention a particular model, I may call him to find out.