so take my response with a grain of salt as I have not heard either of the latest models.
The Dyna 20xh is a big sounding cartridge. I've compared it in my system to half a dozen other carts in different price ranges, and it 'sounds' the biggest. By that I mean, big bass, big lush midrange, and big soundstage. I'ts also smooth and easy on the ear.
The Karat, (i've got a 17d2 MKii) is so different sounding as to make you wonder if they are made by the same company. It's fast and super detailed while remaining smooth as silk. It has deep tight bass, but not as powerful as the 20x. The Karat does sound 'smaller' than the 20x, but then so does every other cart I've tried in my system. I've got multiple armwands on my Scout/JMW-9 so it makes it easy for me to switch between the two.
My advice, buy both!!