I've had the chance to play several C10's and C12's side by side. I prefer the C10 since it has a more attractive tonal quality to my ear. The C12 will be marginally louder and can be driven a bit harder. I wouldn't really consider the C12 a superior instrument, just different, which may or may not suit the player.
The Yamaha GC32 is also an excellent guitar that is responsive, balanced, with good sustain and volume. It has a more clear, slightly less traditional tone which is characteristic of Yamaha Grand Concert guitars.
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening