The two of them form an effective unit, where their strengths each complement the other's weaknesses. Boromir is decisive, and reaches conclusions rapidly once he's got all the facts lined up. Faramir is quicker at assembling the facts, and is able to gather the wider ramifications of those facts when he has them. Boromir is physically dynamic, emotionally stable, and mentally focussed. Faramir is physically stable, emotionally focussed, and mentally dynamic.
Boromir is very good at dealing with the straightforward elements, and once he's got the ducks in a row, he's able to think his way to a conclusion very effectively. He comes across as being something of an intuitive thinker on the battlefield, but this is because he's better suited to that style of action, where the whole mental effort comes down to one simple matter - stay alive and kill the other bugger. He gets relatively easily bogged down in the intricacies of diplomacy (compared to his father or his brother - compared to most other people, he's a capable diplomat) and finds it confusing, simply because he's fighting to stay afloat in an area where his rather straightforward temperament is a disadvantage. It would take him a long time to integrate a large amount of new information into his master world view, which leaves him at somewhat of a disadvantage during the time that he's attempting to integrate this, as he's functioning with an incomplete picture. Once he's got everything sorted out, though, he's able to fit all of the elements together very securely, and thus he doesn't appear to be uncertain of his decisions. He'd come across as deliberate, but decisive. Boromir, mentally, is very much focussed on the concrete, the immediate, and the physical. Once Boromir knows something, it's basically a stone in a wall - it stays fixed where it is, and other facts are added in around it. Note: this does not mean that Boromir is stupid. Quite the contrary - he's actually a rather intelligent man. If he were stupid, he'd be dead by now. However, he's got a different style of thinking to either his father or his brother - he and Findulias probably shared a mental "style", in a lot of ways.
Faramir, by contrast, would be seeing the options inherent in each piece of information, and linking all the information into a mental map. He would be far quicker to integrate new information, and to function regarding all of that information, and would seem to almost intuit the big picture. However, he may come across as occasionally indecisive, as he can see too many options coming out of any one particular path to choose clearly before each of these options has been considered. Diplomacy and subtlety would come to him easily, however he would possibly seem somewhat aloof and distant in battlefield situations, because what would actually be happening while he was fighting would be a lot of background processing. He would come across as wily, but uncertain. Faramir, mentally, is very much focussed on the abstract, the past and future, and the intellectual. Knowledge, to Faramir, is more of a collection of pieces in a kaleidescope, where the various pieces can be shaken up to form new patterns, and each pattern is unique. In many ways, Faramir would be sharing a mental "style" with his father, and this is likely to be at least part of the reason why the pair of them do not get along too well.
The two brothers together would present an effective unit for handling the Stewardship of Gondor, as Boromir would be well suited to handling the day to day petty realities of economic and political wrangling on the small scale, as well as providing a direction on the larger scale. Faramir, meanwhile, would be an effective diplomat, while he would also be able to provide the details for future planning.
Effectively, changing Boromir's mind is a bit like turning around a supertanker. It's going to take some time and a lot of turning space. Changing Faramir's mind can be accomplished by presenting your information. However, getting agreement on a decision out of Boromir would be a quick thing, while getting agreement on a decision out of Faramir would require a bit more time, while he second-guessed his way through things.
It would be interesting, given this, to postulate what might have happened had Denethor decided that his two sons would form a more effective team if they both went to Imladris...