Thanks for all your replies. The thrust calculator is really great, and proves the 12x6 is the best for power! But I still love the 13x6 prop, the plane is so quiet, not screaming like the 12x6. And somehow even after tuning for max rpm, the 13x6 leaves a nice smoke trail behind, hehe, must be incomplete combustion. Many thanks again.

p.s. Here's an on-board video of my plane using 13x6prop. Enjoy!

http://www.youtube.com/watch?v=PF1awEIot2A