As per the nature of their characters, Ralph attempts to emphasize the importance of being reasonable in dealing with the idea of the beast while Jack chooses to respond emotionally. Ralph's dedication to reason is, in the end, overcome when he goes with Jack in search of the beast and "finds" it.
I agree with the above posts and would add that Jack uses the concept of the beast as a means to control the boys and wield his power as hunter/therefore protector over the boys. It is a means to undermine Ralph's authority as the elected leader of the boys.
Ralph reacts to the idea of a beast the same way he responds to Jack's becoming increasingly more savage--he hopes it's not true or at the least not harmful. Jack responds to the beast the same way he responds to everything--he is aggressive and ready to kill, no questions asked.
I think that some people like to ignore evil and pretend it does not exist, and other seek it out. Ralph is innocent, both because he does not acknowledge the true power of evil and because he falls victim to it. Jack is attracted to evil, and to power. He eventually turns to evil because he cannot resist the temptation for power.
The recognition of evil is at the heart of this novel. That Simon is unable to communicate his recognition of the beast leads to Ralph's dismissal of this beast as any real presence until the very end when he is being pursued, and that no one can give a definitive description of "the beast" leads to Jack's attempt to mask himself from the evil he knows exists.
When the kids first come up with the idea of the beast in Ch. 2, Jack wants to hunt it and Ralph wants to talk logically and convince the kids that it does not exist (which it doesn't).
To me, that shows that Jack is the more aggressive of the two and Ralph is more inclined to be a thinker. Jack's solution to everything is to attack. Ralph tries to be more "civilized" and to solve problems verbally rather than through violence.