Here are my thoughts from a Miele owner...
Well, for starters, Miele in other countries differs greatly. In the UK there are no versions available with a power nozzle - you can buy a rechargeable "cordless" power nozzle but Miele would love it if you bought their S7 for that cleaning task.
The Miele S5 in the UK is still selling even though the S8 is supposed to replace it.
I own both - an S5211 Red Pearl vacuum and an S8340 Ecoline model. I previously owned an S6 Ecoline model and an S6 standard model.
The S5 replaces older Miele S700 cylinder vacs, particularly those with the bigger GN dust bag known as XXL models.
The S5 is the old flagship of the Miele vacs and thus it is heavier and has tools that you would hide under a flap on top of the body. It also uses the bigger GN style 4.5 litre dust bag.
The S6 was supposed to replace the old compact S4, so it uses the smaller FJM dust bag (3.5 litres) and does NOT have a flap under the top hood to store the tools - the three of them sit on a clip that can be unlocked and either locked in at the hose on the top of the vacuum or behind the handle.
Certainly if you want a compact super efficient vacuum, the S6 with its smaller bag will serve you well - and it also features Miele's new auto cord rewind known as "Comfort rewind," that basically requires just a tap ONCE on the pedal and the cord rewinds back in.
Personally I wouldn't bother asking anyone in the trade - they will only give you an opinion of what sells and what they might want to sell you!
The new S8 is a bit lighter to lift and pull around than the S5 in my opinion. The S8 also benefits from the auto cord comfort rewind function - one touch or tap from the foot means the whole cord goes back into the machine. The S5 requires you to keep your foot on the pedal.
Some S8 models also come with a touch longer cord than the S5. Ask me if you want to know more. I am not a dealer, but a true Miele owner.