Actually, none of the above
The heat mostly comes from the pumping action. As hard as it is to believe, the water actually heats up, as it's submitted to frictional forces, through the various restrictions within a loop.
The only thing you can do, is stick to flow speeds that are less than 3 fps (feet per second) within the tubing, so that the pumping action isn't wasted. 1/2" tubing usually does the trick.
The pump you have in mind is just fine.