X versus Felix
The thing about the X versus the Felix in my experience is that the Felix is not a lightweight vacuum cleaner. It isn't designed to be as light as an Oreck but it was designed to offer something different when it came along. More like a small to medium flat and home vacuum compared to the X.
The X series, as we all know is pretty old but for a company who are still private and independent, SEBO have just allowed the old design to soldier on with only a few basic upgrades. Only buy an X1.1 or X4 is you want longer lasting dust bags and a lighter weight to push around. Whilst the X can cope, they're good if you want progressive slow cleaning around your home - I find the Felix is far quicker and dependent on the carpet height setting I have it on, can be lighter to push.
Rather than pay out massively for an X1.1 or X4, my best opinion however would be to find or seek out a refurbished X1 Automatic - the dark grey and light grey contrasting models are powerful enough, even if suction is a lot stronger from the Felix.
It's still the model of choice given their cheap prices second hand. In time the X1.1 will take over but I suspect even with the newer models coming in for the EU Law, it may take a long time for the original X1 Automatic to slowly disappear. At least if you try an X1 Automatic, you can either keep that for the long haul or sell it on cheaply and then finally buy an updated X rather than going full hog and spending some £150 or so on a new model.