Personally...
...I think the S7 blows any Sebo product out of the water! And I say that having owned and used extensively Sebo X1, a Sebo Felix, and a Miele S7 AutoCare. And I use a Sebo X1.1 every week.
The Sebo has that stupid short hose, mounted at the top of the machine; you can't move away from the cleaner without it toppling over, unless you keep one hand on it at all times. You can pull the Miele from across a room by its hose, and it won't fall over - it'll just follow!
The Sebo's plastic wand is heavy and unwieldy - the Miele's is much lighter, despite having an aluminium, telescopic wand. And the Miele's hose comes from the base of the cleaner - it won't fall over, and it'll reach all the way up a flight of stairs without the need for an additional extention section.
The Sebo makes up for its mediocre suction by using a very aggressive brush-roll, which the company warns is not suitable for wool or looped-pile carpets. The Miele doesn't have this problem.
The only advantage I see the X4 having over the S7 is the weight - but I can't say I find the Miele difficult to use at all.
I know Which? results aren't always totally reliable, but in my opinion, they've got it almost dead-on. The base S7 model is similar in price to the X4, and you get so much more for your money. The headlight and auto power sensing on the more expensive S7 variants are great, but not essential. And the base S7 model is now available in sleek black, as well as that garish custard yellow!