its faster, can carry more people, MUCH easier to get on and off fully loaded, comfier, roomier etc etc.
I used to be thredders when we got shipped out in a sea king or yank black hawk because it was a pain to get off and retrieve all our tool boxes. The merlin you can literally get up, grab your boxes and leg it off the back off.
that said the worst helicopter ride i have had is on a black hawk. the americans feel the need to remove all the window panels, i was sat back right next to the window for 30 minutes in the pitch black getting blasted with airand kept getting whipped by the seat belt next to me. i also sh!t myself at the speed her came down and hit the deck