Considering your love for the older S100 (a pretty classic number) you'll find the iS96 very much in a similar performance "window". Any actual volume differnce in nett terms is hardly comparable, because the S100 carried a lot (maybe a little much) volume in a fairly thick front nose section, which (in almost all sailing conditions did nothing special), and the slimmer iS96 carries it's full 96 (actual) Lts compliment with a far less "nose heavy" distribution, meaning that all the volume in the 96 is really working where you want it, and none where you don't.
Although the overall width and outline are much more similar between S100 and iS96 than the S100 and iS101, the iS96 still offers some advantage (over S100) in the lighter or gusty end of the board wind range based on the wider tail area and stance. You'll understand that aspect already from the iS101, just scale it down a bit for iS96, but keep it above S100 and you'll be spot on. In comparison, for an experienced slalom guy the iS96 shouldn't be seen to have any less bottom end than the S100.
While the S100 had a pretty sweet drawn out carve style around the bends, the iS tends to improve on "carry" (or glide) thru the lulls, holes and turns, can handle more power going in/thru/out the bends.
Rails on the iS96 are not quite so sharp (in both shape and ride) and that cuts both ways, general average seems to favor the easier speed of the iS over the S100, although "old school" slalom fanatics (pun!) are less fazed by the slightly more demanding trim over/thru chop of the harder railed designs- and some die hards really like the hard engaged feel of the older rails, even if the speed is no better (or potentially more difficult in challenging conditions).
iS96 does not feature Dual flat bottom concept; that is again not always a downside, as to work really well DFC has to be really well dialled (in both design and on water trim). Again for guys who are already "acclimatized" to older flat rocker shapes such as the S100, well the iS96 definitely carries forward that style of ride (with new ease). By comparison, DFC trim really requires the board (rider) to be rockered well back onto the tail block of the board (not as in S100) to ge tthe most. That itself is not that demanding, just different, and I guess in summary, S100 certainly wasn't DFC (shape nor ride style) either !
Hope that helps a better comparison of the options !
Cheers ~ Ian