The S2 is the nicer of the two. Cane Creek describes the difference between the S5 and the C2 as follows:
The S-5 and C-2 headsets look similar. What's the difference between them?
While the S-5 and C-2 headsets share the top-to-bottom configuration and overall concept, the insides are different. The S-5 uses top-quality spec all the way, while the C-2 uses more economical materials. Among the S-5's top-of-the-line goods are angular-contact cartridge-bearing assemblies (with a full complement of continuous-contact stainless steel balls), waterproof grease at a 100-percent fill rate, neoprene seals, stainless steel races, CNC-machined 7075-T6 aluminum cups, and an O-ring seal in the upper cover. On the C-2, there's a balance of high performance and cost efficiency, with the design executed with aluminum cups, steel races and replaceable cartridge bearings. One other thing in the visual department: The S-5 comes in six colors that let you match bikes and forks.
Specs for the S2
11/8 inch (28.6mm)
material: cups. steel crown race with integrated rubber seal. replaceable sealed steel cartridge bearings. neoprene bearing seals
stack height: 26mm
weight: 116g
color: silver, black
U.S. msrp: $50
Specs for the C2
Cartridge bearings, interlocking protective cups.
weight: 140g