Flip a coin. Some guys might claim favor for a one piece mount and rings, but in reality, we're talking about a cantilever mount on a shotgun - not a 1,000yrd bench rifle. It's very likely that you'd never be able to tell the difference in your groups between the two. I'd tend to lean towards a 1pc, but I wouldn't sacrifice optic mounting height to do it.

A couple advantages to using straight up Rings vs. a 1piece mount:

1) You might be able to fit a 3rd ring for added grip. Sluggers throw recoil, an extra ring might not hurt to stop your scope from slipping.

2) You can position the rings more freely to support the tube/scope. Certain 1pc mounts don't always cooperate as well as you'd like them to with certain scopes.

3) You MIGHT be able to get a lower optic height with rings vs. certain 1pc mounts. That all depends upon your scope adjustment housing shape and size and your objective bell diameter vs. your cantilever height and barrel, but in general, I always end up needing a cheek riser whenever I scope a shotgun, so adding a taller 1pc mount would exacerbate that issue.

But in theory, a 1pc mount is a more rigid mounting platform for accurate shooting - if you're shooting in the 1's or 2's with your slug gun, maybe that would matter.