For luxury / creature comfort / mod cons, as you put it, you want as much space as possible.
I'd argue that the real selling point with the G650 and Global 8000 (and current-generation G550 / Global 5/6000 (GLEX)) is the speed. Nice cabin, sure, but you're flying at M0.9, overtaking schmucks in Challenger 850s / Citations (not X ofc) and, for long sectors, beating Falcon 900 / 2000s by a decent margin. An hour less in the cruise is important to some people.
This, combined with nice cabin, long range, and good RWY performance adds up to quite a package, which is why they're so popular.
However, if all you're interested in is the cabin experience (no bad thing - for some pax this is all they care about), I'd argue that bigger is better. 767 / 777 / ACJ / A340 / BBJ / A380VIP (there was one in completions in Basle a few years ago, which I think @GLF alluded to) / 747 VIP (-SP, -400, -8I) / MD-8x (e.g. M-SFAM)
Crazy stuff like baths / showers / hot tubs, full-width retractable rotating dance floors*, massive speaker systems with huge TVs / projectors, sofas, private bedrooms with double beds, ensuite bathrooms and passing corridors, full galley service, business class seating for entourage / bodyguards / press / goons, gyms / exercise stuff, and miniature garages for cars / bikes are only found on airliner-class machines.
And then there's the ramp appeal / prestigious experience of boarding your own personal airliner, instead of a smallish bizjet.
examples of some AOC operators with such birds:
Royal Jet
ExecuJet
Comlux
Global Jet
I think there may be others available for "lease" (bring your own pilots).
*This really exists on a 777.
