For same price the bigger size is better.
But for grands I wouldn't consider sizes lower than 5'8".
This is not accurate at all>
1) The most valuable aspect of a small grand piano, from a quality maker such as Yamaha or Kawai; it is the performance of the action. Students truly benefit from learning with a grand-piano action.
Priority: Choose between these two pianos touch is very important.
Yamaha action has a touch ratio in the GC1 of 6 to 1
Kawai action has a touch ratio in the GL20 of 5 to 1 and the mechanical parts (Non-acoustic parts ) are made of a composite material.
The technical information should be irrelevant to you; but the touch is different between these two pianos, and you should find which one you like best.
2) The sound between these two pianos is also very different, and not because of the one-inch difference in size; specifications for length are based on the cabinetry and not the scale-design, one inch can easily be added to the cabinet.
What is really important is the hammers and soundboard system in these two pianos. The scale design is pretty close.
Again the technical information should be irrelevant, but You should find preference in tone.
3) These two pianos are good quality and the cost of manufacturing is about the same (if not slightly lower in the GC1 ) the price should be very close for any of the two pianos.