There are many diagnostic tests to use when doing all possible regressions. Adjusted R2, Mallow's Cp, and Residual Mean Square are commonly used. All of these three methods can be used in tandem. It is not necessary that these three diagnostics give exactly the same answer al beit they all come in handy.

Let p= number of parameters in model.but if I have 5 variables and a choice between a Mallows Cp of 5.1 and a Mallows Cp of 2, which equation is best? The one with the same number of variables, or the one with the lowest statistic?

Mallow's Cp statisitc is basically a measure of your prediction error (or bias). Models with small bias will fall near or below the Cp=p line, whereas model with larger bias fall above/way above the Cp=p line.

So, I would choose a model whose Cp value is approximately close to p.

In your case: the number of parameters= 5 variables+1intercept=6

So, p=6. The Cp value of 5.1 is very close to 6 than Cp value of 2. Hence, I would chose the model with Cp value of 5.1.