grappling with the Gi is much easier b/c you are going at a much slower rate because either one or both opponents can slow the pace of the match by simply grabbing onto the opponents gi and holding on.
Also, you're technique doesn't have to be perfect, as you have "handles" that you can grab onto for every single technique, in order to steady yourself, to hold your opponent, or to apply leverage.
No GI is a much faster, strenuous game, that suffers much less mistakes. Your opponent is slippery and wet and you have nothing to control him with except good technique, brute strength, and your wits.