Threesome is the best. Easy to maintain the band, arrange jamming sessions, and to sound check.
IMHO, bands with single guitarist are often more innovative than with dual guitarists, they're arent that restricted to powerchords eg. RHCP, audioslave/ratm. Instead they do alot of riffing. Without the second guitarist, the bassist would also be given more room to show off his ability.
Dual guitarists will usually mean 1 guy will do all the powerchords, the other one do lead. It wouldnt be very practical for guitar riffing, because most of the time the lead wont go very well with the guitar riffs. And if both guitarists do powerchords or palm mute powerchords.. chances are u cannot hear the bassist.,And sometimes either of the guitarist will have nothing to play for a very long part of the song.With the powerchord+ lead combination, Your songs will get kinda dry after a while
I dont like the idea of having a keyboardist.. Keyboards sounds too 'processed' and polished for a rockish sound