Fast-casual restaurants are a booming category, and companies like Chipotle Mexican Grill (NYSE:CMG), Noodles & Co. (NASDAQ:NDLS), and Panera Bread (NASDAQ:PNRA) are offering tasty growth potential for investors. Which one should you pick from the menu?

Red-hot Chipotle
Chipotle Mexican Grill is arguably the most remarkable success story in the fast-casual restaurant space: The burrito maker has compounded sales growth at an impressive 20.3% annually during the last five years, while earnings per share grew at an even higher 32.7% per year over that period.

Even if growth has slowed down a bit as the company grows in size, Chipotle continues firing on all cylinders. Revenue grew by 18% in the last quarter, fueled by a strong increase of 6.2% in comparable restaurant sales, and earnings per share increased by 17.2% versus the same quarter in the previous year.

Chipotle opened 37 new restaurants during the quarter, bringing the total to 1,539 locations, and management expects to open between 180 and 195 restaurants in 2014. Considering how strongly comparable-store sales are performing, new openings are not cannibalizing sales at existing stores, and this means that Chipotle still has plenty of room for expansion. Besides, international markets are still practically untapped.

This red-hot Mexican restaurant comes with a spicy valuation; Chipotle trades at a P/E ratio of 53.8 versus an industry average of 29 according to data from Morningstar. On the other hand, premium growth deserves a premium valuation, and Chipotle is no average restaurant by any means.

Noodles got cold
Noodles & Co. priced its IPO at $18 per share on June 27, but investors were hungry for this fast-casual chain serving classic noodle and pasta dishes from around the world, so the stock exploded to almost $52 per share in the following couple of days. Gravity has caught up with Noodles, though; the stock is now trading in the area of $39.40 per share.

With 345 locations, Noodles is much smaller than Chipotle; this should mean higher growth for Noodles as the company is expanding from a smaller base. However, that is not the case when it comes to sales; Noodles reported a 15.4% increase in revenue for the last quarter, while comparable restaurant sales grew by a modest 2.4% on a systemwide basis.

Adjusted EBITDA increased by 24% during the quarter, and adjusted net income increased 44.6% to $3.3 million. Management raised full-year guidance for company-owned comparable restaurant sales growth; Noodles is expecting comparable sales at company-owned restaurants to increase between 3.25% and 3.75% during 2013.

The company is actually doing quite well, but valuation is looking too excessive at a P/E ratio of almost 180 for Noodles. For a lower valuation, Chipotle can offer more reliable growth, particularly when it comes to the crucial metric of comparable restaurant sales.

Panera for opportunity hunters
Panera Bread has enjoyed a leadership position in the fast-casual bakery-cafe restaurant category over the last several years, and this has produced big rewards for investors, as the company increased sales at a 14.8% annually in the last five years while earnings per share expanded at 26.9% per year over that period.

But things seem to be changing lately: Panera delivered a disappointing earnings report for the last quarter, and the company also reduced guidance for the rest of the year. Sales increased by a respectable 8.1% during the third quarter of 2013, but systemwide comparable net bakery-cafe sales rose by an uninspiring 1.3%.

Founder and co-CEO Ronald M. Shaich admitted during the press conference that the company is having trouble keeping up with service quality as it becomes bigger:

We've directly surveyed our customers, and the top reason they cite for coming less often is the diminished in-cafe experience. More than a quarter of those polled identified slower service, less comfort and the accuracy of orders, among other experience issues, to explain why they aren't visiting our cafes more frequently.

Recognizing the problem is the first step to finding a solution, and there is no reason to believe Panera can´t continue thriving in the long term. Besides, Panera trades at a P/E ratio around 26, materially lower than the valuations assigned to other companies in the industry.

Investors looking for an opportunistic purchase with a long term mentality may want to consider the possibility of buying some Panera, as the company is still cheap while it works through its "growing pains" problems.

Bottom line
Chipotle is the undisputed growth leader in fast-casual, and the company is still performing extraordinarily well. Panera, on the other hand, is experiencing troubles lately, but a transitory slowdown could mean a buying opportunity for long-term investors. When it comes to Noodles, valuation is looking a bit excessive for a company that has not yet proven its ability to maintain elevated growth rates over time.

Fool contributor Andrés Cardenal has no position in any stocks mentioned. The Motley Fool recommends Chipotle Mexican Grill and Panera Bread. The Motley Fool owns shares of Chipotle Mexican Grill and Panera Bread. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.