The biggest factor is the band conditions at the moment, and that's all but impossible to predict. Over a 150 mile path, 40 may be your best bet, and 10 minutes later, it'll be 80 meters. Further, as the sun spots increase, there will be times when 20 will be the band to use. At the peak of the cycle, even 10 meters might be the band of choice. In other words, there are no pat answer although a lot of folks think there is.
You're also going to get a few responses about bending over the whip to make a NVIS out of the antenna, but that idea is hogwash. The real formula is buy the best, make the overall length as long as you can, and install it the best way possible (it is the mass under the antenna, not along side that counts).