I read in a magazine that it's easiest to do 3D with a 40% size plane. Why? because the bigger they are the better they fly. The bigger airplanes are more true to scale and fly like there full size counter partners. As far as what model I would say either a Cap 232 or the Edge 540.

The worst draw back to 3D is the $6000.00 min to the $12,000 price tag on the best plane you can get. This price range is RTF of course. You can obvioiusly get things done a lot cheaper just depends on how far you wanna go. You can build a nice Aerotech Edge 540 for about $5400.00 RTF, I know this because a guy at the field just finished one. Just my opinion and 2 $.