I think there are cases that one of them sounds more natural depending on the situation. (In my opinion, たすける sounds more serious than てつだう.) Ex.) If someone help you: with your school homework --> てつだってくれてありがとう。 with your study not to fail your exams --> たすけてくれてありがとう/ てつだってくれてありがとう。 to say something in Japanese / to teach some Japanese words --> おしえてくれてありがとう。 If someone support you: when you are playing soccer against the other team --> おうえんしてくれてありがとう。 when you are in the tough time --> ささえてくれてありがとう。 "Help" and "support" has several translations in Japanese, so please feel free to ask here when you have doubt. I hope this was helpful.

If you want to say "Thank you for your support." then I am pretty sure it will be this: おうえんしてくれてありがとうございます。 You can add いつも～ at the beginning of the sentence to mean: "Thank you for always supporting me." Hope my answer was helpful too. ^_^

thank you very much for your answer! :3

Both are correct. While てつだう only has the meaning of "give a hand", たすける covers almost the same as "help" in English does.