Energy is a product of projectile weight and velocity. Penetration depends upon the sectional density and hardness of the projectile, along with the impact velocity.
As an example, a hardcast handgun bullet impacting at 1100 to 1200 FPS will often out penetrate a softpoint rifle bullet impacting at a much higher velocity. The rifle bullet has a much higher energy level than the handgun bullet. All things being equal, a solid bullet will out penetrate an expanding bullet, even if the bullet diameter, weight, and impact velocity are the same.
Never hunted with shotgun slugs, but I do know the Foster type slugs are pretty soft lead, and will expand to a degree. I would guess that the harder slugs, like the Breneke, or some of the saboted slugs would tend to out penetrate the softer Foster. With a shotgun, I would take penetration over expansion. I DO hunt with cast bullets, and round balls. My cast bullets don't expand, but they are usually of at least .430 diameter. Animals die very quickly with a hole this size in the right place. I imagine they will die quickly with a .720 diameter hole as well.