Mark, I decided to go with the additional ribs because in searching the way back forum threads I noted that some folks had issues with the nose skin deforming with changes in humidity. I live in the Deep South, so I thought it would be worth the extra effort. Not the it gets humid here........
I made 1" thick foam ribs, 2 per "bay" between the plywood ribs. I did this first of all because I was done making wooden nose ribs and did not want to tool up to make more, and I knew I could just glue the foam rib blanks in place and cut them to shape using a hot wire against the existing wood ribs, quick and easy. Weighs virtually nothing and really does stabilize the skin.
That said, if you go with foam, keep in mind that what you use to seal the wing may react adversely with the foam. I simply sealed the wood first, then glued the foam in place, so no problem.
If you decide to install the extra ribs, just use whatever is cheapest/ lightest/ availablist, (Izat even a word?) and it'll work fine.
