This has always been an interesting aspect of the book to me. Faramir was indeed of 'purer' blood than Boromir in his descent from the Men of the West (genetics can do weird things -- my sister and I are more different than you would believe
). We're also told that he took after his mother, while Boromir took after his father (so what does that tell us about Denethor?)
I find this interesting, because the 'pure' blood that is Faramir's comes from his mother; he is also a good fighter, but not a big military hero like his brother, not so consumed with glory and thoughts of battle. What's more, his primary function (thematically) in the book is to be passive -- to let Frodo and Sam go, to NOT kill Gollum, to get hurt by the Nazgul so Aragorn can heal him, to spend time in the Houses of Healing with Eowyn.
The more you look at Faramir, the more feminine he becomes to his brother's masculine. Is his more easy renunciation of the Ring associated with femininity??