If I remember correctly, and assuming they are the same differences as the G15 and I15, the G line is game improvement and the I line is the "tour" line
This is partially correct.
G Series is for Super Game Improvement - Very large soles...thick topline...a good amount of offset...everything that a beginnning golfer would need...Very forgiving...Some use on Tour (See K.J. Choi...a few times)
I Series - Targeted towards the mid to low handicappers...progressive offset...thinner topline...thinner soles than the G series...Less forgiving than G series, but more workable...Some use on Tour (See Mark Wilson - i20 ; Lee Westwood - i10).
S Series - Targeted toward low handicappers only...minimal offset...very thin topline...blade-like soles...less forgiving than the I Series...Most use on Tour (See Hunter Mahan - s56 ; Bubba Watson - s58 I think)
Lastly, AgInMass...your blanket statement regarding the loss of ten (10) yards may be one of the most ill-informed posts I've seen on here in a while, based on a sample size of one (1).