It's not just the speed difference, it's the traffic management policies.
The new 100mbit policy will throttle speed down when you've downloaded so much over the course of the day.
The 120mbit policy will be like the current 100mbit policy that is not throttled (although there is supposed to be shaping on p2p).
Do you have a source for this?
As, as far as I am aware, there is not management on my 50Mbps connection.
The only thing with my current connection and even the 100Mbps is the P2P throttling.