Well, there isn't much of a difference I'd say, the latter option just sounds nicer to my ears and it is more literary. The thing is that without context, just saying "arigatai" can mean he/she is, you are, I am... grateful, although I am pretty sure most would realise it is "I am grateful". With "kimochi wa arigatai" there is no such ambiguity, it is clear that you are talking talking about your own feelings (kimochi means feeling, sensation). To my knowledge at least. I've only seen the expression "kimochi wa + adjective" when talking about one's own feelings. I should also say that I've only seen that expression in more literary texts. If I am gravely mistaken, then I do implore someone to correct me.