Ah. Well Im not a fan of SEBO's electric hose - i had several problems over the years with the K3 Premium and SEBO are well aware of it. I've trialled the D4 some time ago at a John Lewis store who got one in for a buyer and well it is good but the hose on it isn't to my liking. The hose without the electrics added is long enough on the standard "suction only" D2 models, and that's enough for me.
The Felix however is a far lighter machine to use and far more compact even though you still get a long cord, it isn't as long as the D series and of course a smaller dust bag capacity. I'd only consider getting the Disco head if I had more hard flooring in the home.