Yum! Brands' (NYSE: YUM) Taco Bell and Chipotle (NYSE: CMG) both offer quick-serve Mexican fare, but many of us probably find the two restaurant concepts' vittles fairly different. However, Taco Bell is trying to bridge the gap by offering more gourmet fast food on its menu.

These new Taco Bell items will be identified by the name "Cantina Bell." Some Cantina Bell ingredients ring a bell for Chipotle: cilantro rice, black beans, marinated chicken, corn salsa, and pico de gallo. Even more Chipotle-like, Taco Bell will serve up burritos and bowls, with the chicken or veggie versions priced at $4.79 and the steak version available for $4.99.

Both companies downplayed the rivalry. Taco Bell said it's responding to customer tastes; apparently consumers aren't as into the wham-bam of fast food anymore and are now looking for "new tastes and textures," a Taco Bell spokesman told MSNBC.

Still, the fact that Chipotle has nibbled at some of Taco Bell's market-share dominance probably has something to do with this menu addition. Although Taco Bell dominates about half the market for "limited serve Mexican" fare and Chipotle only has 16.4%, Chipotle has been whittling away at Taco Bell's share with just a fraction of the number of restaurants Taco Bell has. (Taco Bell has about 6,000 restaurants dotting the landscape, while Chipotle only has about 1,200.)

Yum! Brands, which also owns Pizza Hut and KFC, belongs solidly in the fast-food companies category like McDonald's (NYSE: MCD), while the Chipotle brand sets itself apart from that category. (Granted, at one point, McDonald's owned a stake in Chipotle.)

It'll be difficult for Taco Bell to shed its downmarket fast-food image, even if it spruces up its menu items. Plus, Chipotle's secret sauce includes ingredients that will be very difficult for Taco Bell to replicate, like Chipotle's "Food With Integrity" mission, which means Chipotle tries to put as many natural, sustainable, antibiotic-free ingredients in its foods as possible.

A Chipotle spokesman told MSNBC, "What makes Chipotle work is more ingredients from sustainable sources with traditional cooking methods in a comfortable environment. There's a lot of soul behind the menu. ... It's encouraging to see restaurants like Taco Bell improve what they're offering."

Indeed, Taco Bell's move brings to mind when conventional grocers like Wal-Mart (NYSE: WMT) began stocking organic merchandise. It meant companies like Whole Foods Market (Nasdaq: WFM) had been ahead of the curve, and increasing numbers of consumers were becoming interested in putting better or more sustainably sourced ingredients in their bodies.

Chipotle's early drive to do quick food better is one of the reasons it landed in the real-money stock portfolio I'm managing for Fool.com. (Whole Foods Market is in there, too.) Innovative, responsible companies lead the way to better business practices, and Chipotle's carved out a niche that Taco Bell only wishes it could emulate.

Do you think Yum! Brands is a better buy than Chipotle? Do you think its new Cantina Bell offerings will be a hit? Let me know in the comments box below.

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