The K2 would be a production design change of the same mower.

I knew about 'Xenoy' but not 'Nexite'. They're obviously both polymer material, and the 'Nexite' has a limited lifetime warranty where the 'Xenoy' was for five years. Looks to me like they have changed over to the 'Nexite'. I had a 'Xenoy' deck on a HRB216TDA and it was fine deck material, so I would suppose the 'Nexite' is even better. In order to be 'better' to me, it would have to be stiffer without being heavier, and I find no real description of it. One small benefit of polymer decks is the color goes all the way through, so you can buff out scratches, etc. and use a car wax on it if you're so inclined, to keep it looking new. The only disadvantage to a polymer deck is it will melt and burn in a mower fire, which would be probably the least of anybody's worry. With minimal care and precaution, you're not going to have a mower fire, and if you did have one bad enough to melt the deck, the rest of the mower would be trashed anyway.