Basically the NN5 / NN5L was built for those using larger cameras like the Nikon D2X and D3 or Canon 1Ds or MKIII. Also those needing to use a battery grip on the DSLR's. NN5 also accommodate much longer lenses and is approx 50% bigger than NN3 to allow for this.
See also: http://nodalninja.com/nodal_ninja_5_pro_pan_head.html
Features on the NN3 can be found on our front page.
Hope this helps - if not just let us know.
Your feedback is appreciated. We have a lot of new items coming in and working to revamp the website to accommodate the growing line up.
In the meantime if you have any particular questions we are here to help.
ps All NN's are constucted of Aluminum cast metal.