In my experience, sour cream is the best choice for this cake. If you use only heavy cream, the cake feels really heavy and too rich. The whipped cream also doesn't absorb into the graham crackers as well, so it will take longer for the graham crackers to soften. The sour cream lightens up the texture and flavor of the frosting. I also think that the mild tang from the sour cream pairs so nicely with the whipped cream and the graham crackers. Another option is to use Greek yogurt, but make sure it's nice and thick, otherwise the frosting will be too runny.