I am an engineer but this is outside of my area of expertise. My advice to you is to talk to professionals in your area and see what they think. I will say this, flexibility usually offers more advantages than rigidity when it comes to foundations and structures. A young tree flexes and survives the hurricane, but the old oak is knocked over. In an earthquake buildings that move survive, and rigid ones do not.
With that as a premise, I would venture a guess that if you dig down deep enough to be below the frost line and lay a thick gravel bed in a uniform manner and power tamp it well, the foundation would be stable year round. It is also likely that if you do it properly, the difference in cost would be insignificant as compared to a concrete pad.
Regards,
Bill