Use the second sentence. A good rule to go by is that we use who for people and that for objects.

You can use either 'that' or 'who' in this sentence, because it's a defining relative clause. There's no difference at all in meaning. You could go for 'who' in this case to avoid the repetition of 'that', but it really doesn't matter. Either one is fine, and 'that' tends to be used more in speech because it fits more easily into spoken sentences. The only situation where you do have to use 'who' is when it's a non-defining relative clause. In other words, if you can omit the relative clause without affecting the structure and meaning of the sentence, you have to use 'who' for a person. Let me know if you need any further explanation, or if your 'gut feeling' can take over at this point!

They're both correct, and there's no difference. In actual speech, I'd say I use "that" more than "who." http://www.quickanddirtytips.com/education/grammar/pronouns-for-people-and-animals-who-or-that

There is no difference when we speak about people. I know the man who/that can help you.

Thanks!! :-) Gut feeling it is, I will put it in my learning vocabulary.