The bottleneck is at the central office, not the neighborhood network (which the cable companies have generally overbuilt by about ten fold). Thus your speed is affected exactly the same way by other people's downloads regardless of which technology you use to get your Internet access.

And I can corroborate this from my personal experience. My cable service has had more than its fair share of reliability problems usually resulting in slow-downs, but they've never had any direct correlation with times of peak usage.