Im quite happy with my old X1A I have. I dont miss the X4 I sold at all but I think that's because I have my old Felix. I think it just comes down to whatever is in your collection and what matters. I would like to buy the Evolution model though when it finally appears. I feel that would be more important in my home.
I would like to see SEBO extend their range of vacuums though. I've often thought a SEBO bagged hand held /domestic back pack vac would be good - when you consider how popular the Dirt Devil hand held vacuum is that uses a bag, the back pack/hand held with a long cord could be a good seller - SEBO already do back pack vacuums in other countries, but they're part of their commercial contracts.
Previous back pack SEBO vacuums used to use the bin assembly of the X series.