Assuming that you're asking about the differences in both sets of intermediate pads, the differences are as follows:
Face of Pad: Jenpro (P4), Microfiber (18K)
Nylon in leg channel: 420D Heavy duty (P4), 400 (regular) (18K)
Leg Channel Foams: higher density in P4 than in 18K
Also, you can customize the P4's, whereas you can only choose from 3 stock colors in the 18K's.
In a nutshell, the materials that the P4 are made from are of a little better quality than those in the 18K. However, at your age, even if you're playing at an advanced level, the 18K's will be more than durable enough for use 2-5 times a week and they'll last you until you grow out of them.
answered 1 year, 11 months ago