A drum brake sheds heat the best of any options you have for the rear.

In my experience, V-brakes have more stopping power and are far superior to cantis. Yes, you can get new pads and tune the cantis etc, but V brakes bite better. If you have road brake levers, you will need a cable pull adaptor for V brakes.

As far as heat dissipation, I think all rim brakes will be about the same.

Deep V rims are probably better than box rims, due to the increased surface area.