When the Chelsea-based Upright Citizens Brigade decided to open a new theater in Los Angeles in 2005, no one foresaw just how warmly it would be embraced. “From the get-go, it was more successful than anybody had anticipated,” the artistic director of the theater, Seth Morris, said. One established comic, Bob Odenkirk, told him it was the best thing to happen to L.A. in years.

What comedians seem to love, Mr. Morris said, is the theater’s combination of high-quality comedy with, for L.A., an unusually relaxed atmosphere. “Established people can drop by and sit in on a show, or try things out,” Mr. Morris said. “We bring more of that New York vibe: It’s a little bit looser.”

While L.A. offers more chances for fame and fortune, the live comedy scene is more pressured than in New York, and L.A.comedians are always conscious of which network executives might be in the audience.

“There’s a feeling in the room of, ‘There might be someone here to see me, who could judge me, and, if I don’t do well, will have a bad impression of me,'” one of the founders of UCB, Matt Besser, said.

“New York is about street cred. L.A. is about IMDB credits,” a partner at the management and production firm 3 Arts, David Miner, said.

In both cities, there are several types of live comedy, including stand-up, improv, and sketch, and a wide range of venues, from the big mainstream clubs (in NYC: Caroline’s, Gotham, the Comic Strip, the Comedy Cellar; in L.A.: the Laugh Factory, the Comedy Store, the Improv), to bars and coffee shops. The number of venues in L.A. has grown in recent years, but there are still fewer than in New York, and the big clubs tend to be very exclusive.

“Even now, I could get up in more places in New York than I could in L.A.,” Mr. Besser said. “I don’t feel I could go into the Laugh Factory or the Comedy Store, because they have Michael Richards up there instead.” (The Laugh Factory was the site of Mr. Richards’s infamous racist tirade, in response to which the club banned the racial epithet that Mr. Richards used repeatedly.)

Outside the mainstream clubs, both cities have strong alternative comedy scenes. (Alternative comedy is generally more storytellingbased, and smarter and quirkier, than mainstream stand-up.) In New York, the major alternative show is “Invite Them Up,” which is hosted every Wednesday night at Rififi on E. 11th St. by Eugene Mirman and Bobby Tisdale. The L.A. equivalent is a show called “Comedy Death-Ray,” which started a few years ago at the M Bar and has since moved to the UCB Theatre.

A New York-based development executive at MTV, Sam Grossman, said he goes to downtown venues, to look for new talent. “Eventually somebody will pop from there,” he said.

“Popping,” or making it big, means going on television — for which L.A. offers many more opportunities. But there are a handful of major shows that shoot in New York: “Saturday Night Live,” “The Daily Show,” “The Colbert Report,” “Late Night with Conan O’Brien,” and the “Late Show with David Letterman.” These provide their own gravitational pull, which can sometimes draw comics back from L.A.

The creator of “The Bernie Mac Show,” Larry Wilmore, has been flying to New York regularly since last summer to appear as the Senior Black Correspondent on “The Daily Show.” “Doing the ‘The Daily Show,’ since it’s a topical show, it’s nice to keep in tune with what’s going on out there, like how do people really feel about the war and that kind of stuff,” he said.

After the creator and executive producer of “Saturday Night Live,” Lorne Michaels, first saw Bill Hader in a small show in L.A., he brought him to do a show in New York before hiring him. “You look at them where you first see them, and then you want to see them in New York,” Mr. Michaels said. “They’re going to be working in New York: That’s where their future is if they get on to the show.” And the cities are different enough that they produce somewhat different styles of comedy.

“If it’s a one-person show, you’re trying to give a sense of what a sitcom series would be like,” Mr. Morris said. “You want an executive to go, ‘Oh yeah, I could see that being on TV for three years,'” he explained. “L.A. tends to be a little more disciplined. New York is a little looser, a little sloppier, in a good way.”

Mr. Besser said that the audiences in L.A. are different, too. “In New York, there’s more of a vibe of coming to the show purely for the sake of enjoying it. In L.A., there’s a lot of people who are in the industry, and they’re thinking, ‘I’ve seen a lot, prove it to me that you’re good.'”

But Mr. Besser added that there are upsides to the pressure. Sure, there’s always the feeling of, “‘Oh my God, there’s industry out there, and if I do bad, they’re going to hate me,'” he said. “But afterwards, if I do well, it’s like, ‘All right! There’s industry out there, and I did well.'”