Single ring - less friction, but the wrist pin better be located at the right height in the piston or you'll experience severe piston skirt scuffing.

Twin ring - less compression loss from blow by. More friction. A little harder to assemble.

I don't know which is easier to break in. I would think both rings on a twin would break in at the same time, but I don't know for sure.

I never noticed a difference either. Have had both designs in many different engines.

Most ringed glow engines have a single ring. They run fine.