I have not used the MRTX, but they are a LOT bigger than the H3. I have head that they go great in the El Fuego witha small trailer (the MRTX usually comes with one). Unfortunately I haven't used the K2.1s either. The biggest thing about the H3s is their construction. When you hold the fin, you can actually feel the flex in the tip, but at the base they are super stiff. This translates into lots of drive, but with the ability to carry a bunch of speed through turns. The fin feels really smooth when surfing it - neither tracky nor overly squirrelly. Just fast and awesome.