BOSTON (TheStreet) -- PetSmart (PETM) and PetMed Express (PETS - Get Report) have benefited from buoyant spending on animals for different reasons. But now there's been a divide.

Shares of PetSmart, the largest U.S. retailer for pet food and supplies, have jumped 31% so far this year, touching a 52-week high last week, while the Standard & Poor's 500 Index has barely budged. PetMeds, which dispenses veterinary medicine and pet health-care products via phone and online orders, has declined 3% after bouncing off a 52-week low at the end of August.

PetSmart and PetMed Express fed off the pet-products industry's booming growth over the past decade, and have proven recession-proof compared with companies whose customers are the human variety. But PetSmart has fared better as Americans cut spending on Fido's expensive treatments, hurting PetMed Express. Still, PetMed Express has a lock on the pet prescription-drug market, which stands to rebound when Americans have more to spend as the job market rebounds.

Sales of pet products may rise 4.8% to $47.7 billion this year, driven, in part, by the increasing number of households with pets, now at 62% compared with 56% 20 years ago, according to the American Pet Products Association. Consumers have been shifting to more expensive products such as "all-natural" and "weight-management" feeds -- for themselves and their animals.

It's difficult to find fault with PetSmart as an investment, although it carries a relatively high 21.2 price-to-earnings ratio. The company, with a market value of $3.5 billion, is increasing market share in a steadily growing industry, even as it competes with big-box retailers, such as Wal-Mart ( WMT - Get Report) and Target ( TGT - Get Report). And it has a long record of revenue and earnings growth, and makes efforts to reward shareholders.

In its most recent quarter, PetSmart reported revenue of $1.4 billion, a 6.2% increase, and earnings of $48.8 million, a 24% jump. That led the company to raise its 2010 profit guidance for the second time this year, to $1.91 to $1.99 per share, up from an estimate of $1.82 to $1.92, in May.

PetSmart has returned excess cash to shareholders, including a 25% increase in its quarterly dividend, and the recent launch of a $400 million share-repurchase plan.

Much smaller PetMeds, with a market capitalization of $390 million, suffers by comparison, albeit unfairly. Its once-booming pattern of sales and earnings growth has slowed this year, giving analysts grounds to downgrade its stock. It price-to-earnings ratio is 13.4.

PetMeds' share price began to slide after the company reported a disappointing second quarter in June. Sales fell 3.6% to $74.4 million, the result of a pullback in buying by cost-conscious consumers. As a result, earnings tumbled 11% to $7.2 million.

Nevertheless, PetMeds' long-term investment appeal should remain intact, based on its low-cost business model, which has produced steady and strong free cash flow and a five-year annual record of 17% revenue growth, 27% earnings per share growth and a more than 30% return on equity.

PetMeds, doing business as 1-800-PetMeds, also boasts a 6% market share of the pet-medications market, which makes up about 8% of total pet-industry sales. That market share isn't likely to shrink given the company's unique competitive advantage: it's licensed to fill prescriptions in all 50 U.S. states, something its competitors can't match. And that puts it in a position to renew its pattern of double-digit growth when pet owners start filling their wailing pets' prescriptions on time, once again.

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