S-lon and C-lon are two brand names presenting the same cord and thread made by the same manufacturer. As a consequence, the difference resides mainly in the colors each brand selects for its line. You may easily alternate.
S-lon thread is a glossy sturdy weaving thread with almost no stretch. This nylon mono thread may be used for all types of techniques: peyote stitch, netting, wrapping leather, and so on. It is similar to Nymo, but frays less. Use S-lon size D doubled to knot gemstones, as it gets through the small holes, but is tougher than silk. You may even consider to triple the thread in order to get a thicker knot. And as for the latest fashion bracelets in leather, weave your ladder with S-lon D. Enjoy it with dune beads.
Size Aa or 0.15 mm: recommended for seed beads and Delica 15/0 + John James needle # 10.
Size D or 0.25 mm: recommended for seed beads and Delica 11/0 and larger + John James needle # 12.
Size Aa: 4 lb.
Size D: 7 lb.
Consequently, size Aa is thinner and less strong than size D, but when your technique demands multiple passes, you may still want to favor size Aa for seed beads 11/0.
Like for all yarn, please keep in mind that dye lots may slightly vary.