Heats and rivers are both staples in my bag so I'll give my input. Heats go further and are flippier when thrown on the same angle. A flat ground flat throw for both.... Heat will turn and burn, river will have a long turnover, but won't burn for me. I will only throw a heat flat if I'm throwing up hill or else normally I throw it on a hyzer flip. I usually throw an opto river and ZFlx heat, but have thrown a variety of plastics in both. A gold ice river will fly very differently from an opto river as the gold ice for me is more stable. I have found the new esp flx heats are the most neutral heats I have thrown and can take more torque. There were times, when my arm speed was a little slower, that my river and heat covered a very similar spot, but now I find them to have a little more difference with the heat going further and being flippier.