I think one should be enough for future Whonix versions. I was never in love with KDE. My preference though will be based on the following points, before visual considerations, resources consumed and applications:
- Any difference in security considerations between the two? more chances for potential holes, frequency of updates etc?
- When considering Whonix 15 (on Debian Buster?), is there a difference in expected release time between the KDE / XFCE?
- Easier or harder testing and development for Whonix team on either?