Well firstly, IMHO the whole proposition of challenging a bagged vacuum versus a bagless vacuum is just not fairly represented and the Kirby is like comparing an American sized fridge freezer with a standard, smaller European model in terms of size!
The Kirby is the better performer as Mieles7 notes, and if you have the time to push the vacuum around, then you'll get an effective, better all-around cleaning performance. However the appeal of the Dyson in most cases is that the machines are lighter, easier to push and don't require as much effort despite Kirby uprights that have the self mechanism activated. Also the old mantra stands "there's no dust bags to buy/replace," but you still have to face the dirt no matter what Dyson would have you believe.
Trebor - I disagree- Dyson uprights DO lift carpets but their design doesn't necessitate that they are required to deep clean - the suction power on board in most cases takes care of that already. Dyson uprights lack the superior performance of leaving carpets groomed though.
In my experience, a brush roll that may well have a higher spin or RPM isn't always good for certain carpets and the term "over wear" springs to mind here, when too much vacuuming with a higher-spin brush roll can do carpets more bad than good where general condition is concerned. But then it also factors in the kind of brush roll that is used at the time. I liked the brush roll that Oreck XL uprights have - very bushy, not that rough to the touch and yet leave carpets looking good.
