The S two part seat fits on a R and the one piece R fits on a S. Fully interchangeable.
The single piece R seat is p/n 63507140000, about £116,
The two part S seat is p/n 63507040000, also about £116 plus the pillion seat p/n 63507047000, no price available.
For the rider, the S seat is 2cms lower than the R, but it is slightly wider.
There are a number of other seats in the catalog, lower and higher, none of them available yet. There are even heated seats (Ergo heated front P/n 63507940000 and Ergo pillion seat P/n 63507947000).
Rebel X Sport propose a rally seat cover with logos on all sides to put on the R seat. This is a permanent transformation, the original cover is completely removed.