Guest - You're probably right.

The five shapes of CAGED are something that Eric Roche and David Mead go on about quite a lot and they know/knew considerably more than me.

If you play a C - x32010 and a D as xxO232 then they are structured differently; the C is Root,3rd,5th,Root,3rd and the D is Root,5th,Root,3rd so there is some difference. Played this way one doubles the third one doesn't - one has 5 notes the other 4.

If you barre the C and D shaped chords up the fretboard (which I do) there are things that much harder to play with the C chord shape (eg adding the 4th on the top string) than with the D shape.

If you play your D as x54232 or x50232 then yes it is very similar. Don't ask me ask a guitar teacher :)