The more natural option for a person who helps is "to" in my opinion e..g. Thank you for your time. You've been very helpful to me. For things, either option feels equally OK. e.g. The advice you gave was very helpful to / for me. There is a longer construction with for + object person + to + infinitive e.g. It is very helpful for me to listen to the radio, when I want to improve my listening skills.