Warning- My response may upset several fans of the concerned actors. Kindly ignore this response if you can be one of them.
Personally, I feel it would have been greater to not compare the greatness of two actors . They are individual actors and have given many good films in their long career. But, even then, if I would have been forced to choose one, I would have voted for Rajnikant. Not because I like him more as an actor or person, but because ever since I can remember, I have heard more buzz about Rajnikant and his films than that of Mr. Kamal Hassan. In fact, even when I was a kid, I never liked Mr. Hassan playing the role of Chachi or something (I don't remember enough). That film however had have been successful, if I am not wrong. I do not wish to disrespect him or his acting, but for me as a child, it was always a pain to watch when his film was telecast on Doordarshan, the only channel we had at that time.
I have not been even a fan of Mr. Rajnikant. However, I was pretty able to watch a few of his movies that were shot before 2000's, the reason being that we had only DD1 channel and my parents and relatives used to watch somewhat old movies during those times. Since then and till now, I have been hearing more about Rajnikant and as little as nothing about Kamal Hassan. So it is obvious that I see only Rajnikant to whom I can vote if I am given just these two choices.
I have watched movies of 70's, 80's, 90's and afterwards... And I did find several of them entertaining, but sorrily, none of those included these two good actors.
I am sorry if it hurts anybody, but that is all I know about myself and my taste, even if that's a little sour or bitter. I repeat, my confused vote goes to Rajnikant.
"Your value does not decrease based on someone's inability to see your worth"