I have both balls. For me the Horizon is the stronger of the two, a good, strong medium bowling ball. The Heat is more of a medium/light ball, somewhat similar to the Solution Power Plus Ex. Both are good bowling balls. For a medium bowling ball I would check out the Heat Blast from Track. Another good ball, or perhaps the Equation. But if you're choices are the Horizon or the Heat, for medium conditions, I would go with the Horizon. But check out the Equation.
Chuck