The earlier DC14s have brittle ABS plastic, the later DC14s are more durable. It's quite rare to see a DC14 physically broken from normal use (except maybe the hoses that split when people overstretch them)
Dyson learnt their lesson that tough plastic doesn't mean it's the right plastic. Hence why they now make their machines from flexible plastic. Less chance of snapping.
The earlier DC14s have better motors, the later DC14s with YDK motors and DC33 had the crap motors. The DC14 ones were okayish, lasted a few years
Early DC33 motors didn't even last long at all, next door's DC33 (brand new) blew up after a few months
The different variants of DC33 just differ from each other with what they include in the box. For example, the animal has the mini turbine head and the stubborn came with a stubborn dirt brush and dyzolv.
Clutchless ones weren't designed to be used on carpets. All Dysons now have HEPA filtration and brush control, whereas older ones didn't even have them unless they were a more sophisticated variant