I would go used ..not sure what Fantom G6 or Motif XS prices are in the UK used.
For the best of them as a master keyboard, its the Fantom G. I have yet to find anything that is as easy to setup splits & layers, and most importantly, turn on and off zones quickly. The live set view on that big screen is an art in simple and legible UI, and the pads can be used to turn on/off the 16 external channels. The keybed is very very nice. When i am
dicking about making inspiration music with the hardware synths (VIrus, Nova, uQ etc) its all being run from the Fantom
If you want the thousands of arpy type things then the Motif now has Karma, but it is a bit more fiddly UI (perhaps still simpler than the Kronos would be?). Given that the S6 piano they added for the XF is horrid, I would go for the motif XS to save money..unless you expect to gig with it in which case the flash makes it worthwhile.
Soundwise they are apples and pears.. Motif has more natural acoustic sounds strings, sax, piano, but pads, vox, synth bass & lead its the Fantom that has the edge.
Must say though, if Karma was a big attraction then i would go for a used Motif XS/XF