Dad’s short answer: The good ones get within about a millimeter on both sides, which in practice means you're not going back with a rag. Single-side edge cleaning means you have to think about which direction you're pushing, and that gets old fast.

Both sides is worth paying for

Dual-edge means the roller runs nearly flush to the housing on the left and the right. You push the machine along a wall in whichever direction is natural and the edge gets cleaned. Single-side means one edge is flush and the other isn't, so you're constantly turning the machine around to present the right side to the wall.

That sounds like a small thing on paper. In an actual kitchen with an island, cabinets on two walls and a doorway, it's the difference between one lap and a lot of fussing.

Corners are still yours

No floor washer in this category cleans an inside corner properly. The roller is a cylinder in a housing; it physically can't reach into a 90-degree corner. Anybody claiming otherwise is describing a marketing render.

What the good machines do is get close enough that the corner never builds up visibly, so a quick pass with a rag every few weeks handles it. What the poor ones do is leave a visible dirty stripe along the baseboard that you end up mopping by hand, which defeats the purpose of owning the machine.

Dad’s Take: Dual-edge on both sides is one of the few specs in this category that changes how the machine feels to use every single time.