Having been to several different types recently, I must say that I vastly prefer the "circle" approach where the "next on" spot just moves logically around the room from person to person. The "MC" role is very limited in that scenario, but (s)he can still make a valuable contribution by keeping things moving, ensuring that latecomers slot in at an appropriate place, decide when it's time for some "all join in" tunes, and ensure that people aren't aligned to that you have 5 unaccompanieds followed by 5 guitars.

I don't much like the set-up where the MC "gives the nod" to the next person to perform while the current performer's singing/ playing. If the MC knows all the participants well it may work, but often results in someone being missed out, or MC's favourites getting preference.

A good singaround should *appear* to run itself.