daniel is right, although the dimensions are wrong.
the U1 is 120 cm, the U3 is 130 cm and the U2 is somewhere between the two. I think the u3 also has slightly longer keys (on the inside of piano) , possibly giving more control over nuances. The added height of the U3 also gives a better bass.
check yamaha website or google for more info.
im thinking of buying one, secondhand.
you too?
anyone got any tips?