frednz: ... We are about to upgrade our cable broadband to either 50Mbps or 100Mbps. We do quite a lot of high definition video streaming and we wonder whether it's worth paying the extra $10 per month to get 100Mbps instead of 50Mbps? ...
The current peak congestion problem effects both 50/2 and 100/10Mbps cable broadband, in terms of downstream speeds:

(click to view)
These problems will hopefully be fixed in August-September.
Upstream speeds have NOT been affected - so if upstream speed is important to you, go for the 100/10Mbps plan.