You are right in assuming that heptane has the larger BP and your reasoning is pretty good. The larger the molecule the larger the surface area the greater the attraction due to induced dipoles. However, it is really the vapour pressure that is important. The vapour pressure is the pressure exerted by gaseous molecules when in contact with their liquid phase. The lower the boiling point the larger the vapour pressure. The larger the vapour pressure the greater the rate at which you can evapourate the liquid for a given temperature (p is proportional to n because PV = nRT). The faster it evapourates the faster the liquid left behind cools. Therefore hexane will cool faster than heptane