Offizieller Kommentar
The DDJ-S* units are designed for use with Serato, and have now been supported under rekordbox DJ as well. The DDJ-R* units are intended as rekordbox-only controllers, but have seen updates to take advantage of different features in the rekordbox program, as well as improvements to the internals.
The *Z and *X units are different in their size and ability, so it's hard to compare them to each-other. You'd be better off comparing similar units, but again, the primary difference is Serato compatibility.