
I decided to go for Boromir although I tended first to Faramir for he is more valiant and gentle.
Why did I choose Boromir?
- Boromir, like Philipa put it right, was a victim of his father or even more of sauron himself who poisoned Denothor by using the former 'innocent' palantiri
Unfinished tales p.523
Denethor's mind was poisoned by using the palatiri through which he got 'abused' by Sauron. Nobody can say when he first started to use it (it would be around 2988 when his wife died) but there is no doubt in the power of the stones. This is perfectly illustrated by the 'horror' in Gandalfs face when Peregrin looked into the Stone. Nobody thought of the palantiri as keyrole in the war until this point. Gandalf was after that in a great haste because he deemed that Denethor was using the stone aswell.It must be remembered that the Stones (palantiri) were originally 'innocent', seving no evil purpose. It was Sauron who made them sinister, and instuments of domination and deceit.
We must see the stones as less powerful 'rings', were turned rather to control its user than to reveal truth.
To get to the point: Denethor was a absolutly charismatic person as we can see when Pippin meets him first. There is no doubt that he was able to influence Boromir.
Boromir was the first born and with this comes a lot of responsability that would not be for the second born Faramir. Boromir stood under strong pressure to fulfill the will of Gondor respectivelly his father's. Boromir was compelled to think for Gondor and his blood. He fighted his whole life for his people and has never forsaken them. He wanted them to be free and proud and was willing to give everything for that.
Faramir on the other hand was second born and was always protected by Boromir, who had nobody protecting him in return, he had to carry the whole weight.
Faramir was in the easier position. He could be gentle for he was not ultimatly in charge.
That is only the peak of the eisberg.
Let's talk about Boromir's ringlust.
The ring is the most powerful thing still existing in middle-earth, the ring has his own thoughts and plans. The ring choose his carrier or at leasts influence his own journey where he might.
The Ring never wanted to be in Faramirs hand, who would have lead him away of his master/servant (sauron). In the case of Boromir it would have been an advantage for a hobbit is not best for dominating

With all this points I want to make clear why I would have picked Boromir. He was a main target of the evil. The ring awakened a lust in far more powerful and wiser men (Isildur, Galadriel) and still in the end he withstood his lust although it just might have been for a moment and saved all the hobbits.
I would call for him because I see his drama and I would be sure that he would never forsake in defending to his own death!
Who can say the same about Faramir? Yes he would fight till the end for his father and for his men, but would he for a former stranger? I don't know.
Sorry for the long post

some other short reasons:
- Faramir is similar to Tolkien himself. Tolkien was writing in the war while somewhere men were dying and he heeded more trees than men

Please do not misunderstand this is no offend against Tolkien, but when I am in the greatest peril than I want someone who cares more about the human being than for nature (so would not pick me either

So that's it
