Regardless of how durable the Contour was over the 600 series. The 600s were much better in just abould everyway to the original Contours, they had stronger suction, picked up better, had better tools, better bag fitting as it has a clip to hold it in, it also had a propper handle release with an auto nozel raise and not to mention a sturdier lower cord hook.
Just my views but having owned a 612, 616 and now this brand new 614, the later varients of the 600 series 614, 615 and 616 were all sturdier than the original line, which were 610 and 612.