What is the nut gonna hold up bettr to? Tree impacts??? HEHE
These are flat on the back, pick size and color:
http://www2.towerhobbies.com/cgi-bin...?&I=LXA411&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXBMY5&P=7
Try 1 if you like or go with the nut or the 3-blade. Again, make sure you get things tight.
If it were me, I'd stay with the 3 blade a bit longer but it is your choice in the long run.