That's an interesting question, bringing up the idea of what "valuable" means. Certainly to the reader Dunstan (formerly Dunstable) Ramsay's life seems more valuable simply because he is more self-aware than Boy Staunton. Boy Staunton (who started out as Percy Boyd Staunton) never pays for his part in what happened to Mrs. Dempster, and this, at least from the reader's perspective, colors his whole life. Because he was brought up in the staunch Calvinisim of a Canadian village, even when Boy's conscious mind rejects any wrong-doing, and he spends his whole life having success after success in every way that the world approves, his psyche is never at peace. He allows Dunstan to take care of Mrs. Dempster her entire life after her aunt dies, even though Dunstan had already rendered her many services as a boy and as an adult, and, arguably, Percy had more guilt in her becoming "mad" (if anyone could be said to be guilty for that) than Dunstan ever did.
As far as Dunstan "winning" -- Dunstan does nothing of the kind in relation to Boy Staunton. Boy takes Leola Cruikshank away from him, and marries her and tires of her. The fact that Dunstan no longer loved Leola by the time Boy married her makes no difference -- technically Boy "won". Boy has two children, and marries a second time and has a stepdaughter. Boy's parents outlive the flu epidemic. Dunstan loses both his parents and his brother, and never marries. On top of it, in the war in which Boy Staunton postured around as a major, Dunstan lost a leg and part of his arm. Boy makes millions in sugar, and Dunstan, though he has a comfortable nest egg, teaches school and lives the lonely life of a schoolmaster -- not even given the headmastership when he had pulled the school through the war. Dunstan's Victoria Cross hardly seems recompense for all of his sacrifices.
The main difference between Dunstan and Boy is self-awareness. Boy has no time for reflection, and, if he does, it is of the most egotistical, self-congratulatory type. Boy never addresses his poverty of soul until it is too late; Dunstan spends most of his life addressing the poverty of his soul, and trying to reverse it. The two childhood friends have little in common of temperament and character, but only share a similar upbringing and their characters' bases in strict Calvinism. This shared past is what keeps them friends, and perhaps some sense of obligation of both sides. Dunstan admits that he always did like Boy Staunton, though we see the worst of that man's character through Dunstan's eyes.