Bill Mahr is an asshole comic who can't hide the fact that he feels nothing but scorn for people who don't operate within his intellectual sphere, and refuses to make a genuine effort to understand their point of view. However, he respects factuality.
Rush Limbaugh is an asshole comic, and fat white entitled racist pig, who buys entirely into the mentality of the privileged manipulating the plebes for "the greater good, and to assure our destined ascendancy." He lies, distorts, dips below even the asshole level in insulting and mocking virtually anyone, and supports a sociopolitical machine that aims to subvert human progress in the name of profit and the rule of other fat white men.
Both men are dicks, both are fundamentally unlikable, but in the final analysis quite different in terms of what their attitudes are predicated on, their motivations, and their ultimate aims.
Bill Mahr is using his gig to raise awareness of hypocrisy, like a lot of comics. He's just a jerk about it, and hits below the belt at the wrong times, and falls into skirting argumentative fallacies or simplifications because he believes the other side is inherently wrong and he doesn't really have to worry if he's right.
Limbaugh is a troglodyte from the past who is a kinpin in the campaign to deceive and manipulate the ignorant for his and his fellows' personal profit, and to burn the world as long as it keeps their cigars lit. He doesn't actually care who is right or wrong, and sees such questions as the self indulgence of academia. All the matters is the unassailable might of money and power to keep the "right" people, his people, in charge.