Both could be が without changing the meaning. In this case, though, it is possible to use の in basically the same way without changing the meaning. This の is not the same as the の that you learned as a noun modifier, like in 私の家, but rather the nominative case (主格). I am not an expert on grammar, but there is a lengthy post on this subject here (in Japanese). You can also check it on wikipedia in whatever language you desire.
If you cannot read that page, basically it says that this construction comes from a classical Japanese construction that uses the word order ~の+連体形{れんたいけい}+名詞{めいし}, or some object plus a participle plus a noun. The example given on that page is the old phrase 「嘴の赤き鳥」=嘴が赤い鳥が. In the example 心の強い人, 心 would be that object, の is the particle in question (格助詞), 強い would be the participle adjective, and 人 would be the noun.
If I've made any errors here then please someone who is better at grammar correct me, of course!