What advantages do Jack and Ralph have as prospective Chiefs?
Early in the novel, you see the conflict between hunting and keeping the fire going - Jack is for the former, and Ralph for the latter. You see in here, I think, the key differences that Ralph and Jack offer as Chief of the island.
Ralph's approach is undoubtedly the common sense approach. Ralph, thinking long term, is focussed on being rescued, and on getting the boys of the island. This is a clever, and useful thing to do - but unfortunately, it provides no short term returns. It's just an endless watch over the fire, in the hope that one day someone will rescue the boys - and, of course, ironically, it is indeed because of a fire that the boys are eventually rescued.
Ralph's approach to leadership is similarly long-term: rules, the conch, democracy, assemblies, and so on. It's all very worthy, and very grown up - but it bores the boys, and when Jack decides to say "Bollocks to the rules", it's all too easy for them to go with him.
Jack presents an alternative approach to the island. He's not interested in the fire, in the rules, or in democracy - all the things associated with Ralph, and which make him boring. Jack's approach is far more glamorous: the thrilling, heady rush of the pig hunts (which even Ralph is drawn into) which provide a short-term reward to the boys - delicious food. Jack is also impulsive, dark, strong and brooding - a far more glamorous, scary and charismatic leader than Ralph, who is sensible, but dull.
Jack has the advantage of being carefree but with an air of arrogance. This draws the other boys to him because he promises them fun. He scoffs at the rules, which make the other boys defy Ralph because Ralph tries to uphold the rules and tries to maintain order. His ability to provide meat to the group is an overpowering force that the boys cannot deny. Later, Jack embraces the savage inside himself, and the boys are afraid to go against anything he tells him.
Ralph has the advantage of being the first to call a meeting. This gives him the initial power of the entire group of boys. He also uses reason, with the help of Piggy, to keep the boys focused on getting rescued. Unfortunately, this power isn’t strong enough to keep the boys from slipping into their savagery.