We recommend the reinforced heavy duty nylon arms be used whenever possible. They fit tight, have a brass ring to help prevent stripping, and are very stiff. But they still have a little give you you crash, etc. They are a favorite amongst our pattern flyers and heli flyers.
http://www.hobbypeople.net/index.php...sert-2-pc.html
If you need aluminum arms, I recommend the Maxx Products arms because they have a set screw that "locks" them onto the spline. These are popular with our IMAC flyers who need the big 3" arms.
http://www.maxxprod.com/mpi/mpi-98.html