I think grateful and thankful are about the same. So, you can use/say either grateful or thankful. I'm grateful to you or I'm thankful to you, not for you.
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I'm grateful and thankful to you. = I very much appreciate what you did and I thank you very much.
I'm grateful and thankful for you. = I am very happy and I thank my 'lucky star' that I have you as my good wife or husband, for example.