What are the differences between the Yamaha Motif XF and XS? Here are some...
The XF has 128 MB of SDRAM where as XS had none although you can expand your XS with DIMMS to a recommended maximum of 1GB. The XF is expandable via 512MB or 1024 MB Flash Memory Expansion Modules.
The XF can read load and import 99.9% of all data created on an XS but it will not work the other way round i.e. XS cannot read, load and import data created on an XF.
The XS is a just as good a synth as XF is. So, if you are not really worried about buying a keyboard that has been discontinued, you may get a good deal for the pending stocks if available. However, just keep in mind that XF series obviously has an updated hardware which will not be available in XS series.