They are quite similar, so difficult to choose. Military is harder, larger Pivot, much larger washers, and easier to flick and maintain. P4 has a longer blade, cool steel for Spyderco, and tough/non-stainless liners.
I predict, if you like either, you'll end up getting the other one too. So maybe start with the Rex45 Military, as it's a sprint run and still can be had new ($182 at BBS) ?