If you cannot try them in a shop, you should spend some time listening to a lot of demos of Psr/Tyros/Genos on youtube, and you will understand that having a Moxf at home, none of these keyboards will bring you something better in terms of sound/instrument quality.
The best for that is to use virtual instruments (vsti) on cubase, or if you prefer hardware to look maybe at "Yamaha Montage" or "Korg Kronos" (which i am not familliar with).
About adding new samples, maybe you are not aware that you can do it in the Moxf installing memory extension,
https://www.mutec-net.com/product_fmc-567.php?PHPSESSID=80mast2lvpa1qqgo7snmsr8s94&lng=en,
and using this software,
http://www.jmelas.gr/motif/wave.phpEdit: I have been on youtube comparing Kronos and Montage, and to my ears Montage sounds substantially better, at least for the common/standard instruments (pianos, strings, guitars, brass, woodwinds, etc...). For Synth sounds, none is better than the other, it is a matter of personal taste. (I prefer Korg synth sounds).