My guess would be unlimited will not throttle the connection if you are hogging it.
Unmetered will still monitor you for hogging and throttle the speed [lets say you are buying 50 Mb down and you are using it all 50 Mb/s for a day solid you may find you get dropped down to 10 Mb/s for a while] you get as needed to manage their network.
But reality is it's probably 2 ways of saying the same thing and some marketing dweeb thinks one sounds better than the other since they all probably look for those users clogging up their network with non stop downloading.