I'm for the round the room irrespective format. It's easy to run without having to take tally of who arrived when and how many songs they've each sung since. It means that all who wish to perform (even the ones I might not paricularly like!) get a fair crack of the whip.

Regarding the issue of duos, trios etc., this shouldn't really be a problem. I attend some singarounds with my folk band, which is a trio. We generally get one go each. Each go can be taken as an individual (solo or chorus), as a full band, employing only two members of the band or one or more members of the band performing with other performers not in the band. The possibilities are endless! On particularly busy singarounds we often limit ourselves to two or even one to avoid others waiting for ages or even missing out.

As for Marje's comment: "A duo/trio/quartet doing 2/3/4 times the usual quota of songs (together) will get to sound a bit samey", this shouldn't really be the case (note that I said "shouldn't" rather than "isn't"!). If a band can't manage two or three songs without sounding "samey", how would they handle a half/full gig?