I'd do the 5625's. Speed is everything for 3D. The 131oz's of torque on 6V is more than enough for a 9lb plane.

Personally I like fast servos and on the DX7 you will notice that the connection you have to the plane feels loads faster. I went from a 10X to the DX7 on my Favorite 50cc Cap and had to lower the rates and add expo because it seemed I had way better control over the plane.