i think they r both great but honestly GC takes me more by surprise with his emoting than DD. DD is fab but the gamut of emotions that GC expresses through his eyes only-remarkable, i repeat no other tele actor can do that, he is one actor who is so good even without dialogues because his eyes does the speaking always, as far as natural acting is concerned, both r natural actors, but those who feel DD is a little more natural, thats just because her role is sketched that way, Geet is spontaneous and maan is not supposed to be as spontaneous as her, so in certain scenes, he has to put up an attitude, thats the requirement of the role and gurmeet is doing 200 prcnt justice to it,and i think his role is actually more tough to play than geets because he has lot of shades-anger ,sometimes extreme anger,attitude, arrogance,love, pain, hurt, humility-everything-and that too without long dialogues because maan is not supposed to express himself so much, so all his expressions r so subtle and yet u can notice every tiny expression change-simply outstanding.
and most importantly both of them cannot be compared because they r playing totally different roles which need to be played differently, and they r doing a superb job.