trying to decide which one would be better for houdini.

seems like the k5000 would be faster, but the 6000 has 2 gigs more vram.

The K5000 isn't available for another month or so, if that makes an impact on your decision. If you can wait, I think you'd find the K5000 a better value than the 6000, unless you're doing huge OpenCL sims (>300^3).