They basically mean the same. Maybe you'd use the first when the fact that you are unable to help is something quite permanent, "I'm in no position to help you write an essay in French since I cannot write properly myself"' "I'm unable to help you because I'm in a bit of a hurry" But really... I think they have the exact same meaning.

I am unable to help you because: I don't want to help you. I am unable to help you because: I don't have time. I am unable to help you because: Your problem is yours to solve. I am unable to help you because: It is a test, and I cannot cheat for you. I am unable to help you because: I don't give money to strangers. I am unable to help you because: you are dying, and this cancer is stage 4 and incurable. "I am unable to help you" is a polite way of saying "This problem will not get fixed (by me.)" There's a polite admission of unwillingness, which is represented as "unable-to-ness", aka "inability out of one's own control." Whether the help would make any difference, or be of any use, is still questionable. I am in no position to help you: because your knowledge is better than mine. I am in no position to help you: because I am not strong enough to lift that. I am in no position to help you: my leg is in a cast, and I need to stay in bed. I am in no position to help you: I have no money, I am broke, my bills aren't paid. I am in no position to help you: my life situation is worse than the problem you need solved. When I hear this, I think "I am in no (financial) position to help you." It is an admission to being poor at the moment. Poor in money, or poor in resources, or poor in education, or power in power/control (as in, "I am in no position to help you, my boss would kill me." (Kill= be mad) This phrase passes along an emotional burden to the listener. "I'm not really in a position to help out right now: my dog just died, and I have $2 for food for a week. People might at times use this phrase on purpose to appear poor, sad, or not a target for the person's requests. They value the peace and solitude of not getting pressured or asked for help more than they value a positive outward appearance of their life. Imagine a Bill Gates in a Toyota Corolla, saying, "Sorry, don't have money right now. I'm not really in a position to help out" and then drives away.