Easiest way to tell the difference... One says "R6" on it, the other says "R6-S"!!
By all accounts, there's not much difference between the two, actually.
Both are fast, racy and sporty. The S is the Street version, whereas the R6 is designed with the Track in mind. The latter will get up to speed a microsecond faster and that's about it.
Slight differences with forks, frame shape and so on, but for the most part they are the same bike (hence each being called an R6).
R6-S
R6
If you google "R6 vs R6s" or "difference between R6 and R6s", there's a whole host of opinions on R6 blogs, R6 forums, R6 porn sites and so on, most of whom are arguing about details so seemingly minute that you'd only see them if you use a microscope on the cellular level!!