As I was saying in another thread more recently, there aren't that many bagged uprights on sale in the UK at high street shops anymore which IMHO is a great pity, compared to bagless uprights. John Lewis however have reinstalled their Electrolux Hilight bagged vacuum under their own name, which is at least a good contender alongside the Dirt Devil/TTI based uprights or if you have to go Premium, it's a Sebo at JL, Currys, Debenhams, House of Fraser's or Miele at Argos and Comet.
I had a few Powerlites. The belt does go quickly on these because the brush roll is very eager and the height adjustment dial slips easily, if you are determined to believe it can clean hard floors and the back to carpet again and keep changing the height adjustment. I think Electrolux knew that they could sell a pretty cheap vacuum cleaner and to just fit it with cheap parts and pretty flexible plastics. However what I do like about the Powerlite is that it is super light to push, and the top hood comes off via two screws to change the drive belt instead on the base plate like some other brands. The hose is too short but I eventually bought the "Stair master" hose extension from Electrolux to alleviate the short hose problem. Didn't like the injection moulding of the rear bits though and the upholstery brush was of the similar oval style that Panasonic fit on their bagged uprights. I also had the mint green, short lived Eco version of the Powerlite that first came out when the white models were on the market and with only 800 watts, it was quite powerful and noisy but had a tendency to lose suction too quickly despite the use of the white electrostatic dust bags. But I never quite understood why Electrolux never fitted a piston valve bag indicator in the front bin door to ANY of the Powerlite uprights and have since failed to include it, even on the more recent models.
As for Felix and X - there isn't much of a weight difference between them (Felix is 6.8kg against 7.3kg for the X1.1) but the Felix feels lighter because of the ratio of parts and for the fact that it swivels. It has a simpler arrangement where its power head is concerned but it also lacks the double insulated thermal cut out that the X is just fantastic at having, even though the two share the same principle of shutting the brush roll off if something gets caught. When something gets clogged in the tube, or blocks the air flow, X's will switch off automatically until the clog is removed. Then the whole system starts up immediately via the power switch. No cool down time required with the X at all, one of the reasons to why it is so successful as a commercial upright vacuum.
My first Felix/Dart experience was actually with the Dart 1 before I bought my Felix Navy. The only decision I don't agree with Sebo UK is discontinuing the X based "Essential G" / G1/2 series where there's a manual height adjustment dial instead of the auto sensor. Sebo UK's decision to replace the G2 with the Dart never made much sense to me on the basis that the Dart/Felix has the small 3.5 litre dust bag compared to he 5.5 litres on the X/G2.