Round armor is probably better. The problem is manufacturing. You'd have to cast whole sections and repairs would be more difficult. It's a lot easier to stick flat pieces of armor at an angle.

I just wish WWI designs would come back with their goofy-ass shit like 12 person tank crews, tricycle towers, and 8 turrets. We will never see the creation of Bolos because physics is a meanie who says "No" to all the cool stuff.