Investors looking for stocks in the Medical - Dental Supplies sector might want to consider either McKesson (MCK) or West Pharmaceutical Services (WST). But which of these two stocks presents investors with the better value opportunity right now? Let's take a closer look.There are plenty of strategies for discovering value stocks, but we have found that pairing a strong Zacks Rank with an impressive grade in the Value category of our Style Scores system produces the best returns. The proven Zacks Rank puts an emphasis on earnings estimates and estimate revisions, while our Style Scores work to identify stocks with specific traits.Both McKesson and West Pharmaceutical Services have a Zacks Rank of # 2 (Buy) right now. Investors should feel comfortable knowing that both of these stocks have an improving earnings outlook since the Zacks Rank favors companies that have witnessed positive analyst estimate revisions. But this is just one piece of the puzzle for value investors.Value investors analyze a variety of traditional, tried-and-true metrics to help find companies that they believe are undervalued at their current share price levels.The Value category of the Style Scores system identifies undervalued companies by looking at a number of key metrics. These include the long-favored P/E ratio, P/S ratio, earnings yield, cash flow per share, and a variety of other fundamentals that help us determine a company's fair value.MCK currently has a forward P/E ratio of 10.80, while WST has a forward P/E of 58.12. We also note that MCK has a PEG ratio of 1.63. This popular metric is similar to the widely-known P/E ratio, with the difference being that the PEG ratio also takes into account the company's expected earnings growth rate. WST currently has a PEG ratio of 2.84.Another notable valuation metric for MCK is its P/B ratio of 4.60. Investors use the P/B ratio to look at a stock's market value versus its book value, which is defined as total assets minus total liabilities. By comparison, WST has a P/B of 11.35.These metrics, and several others, help MCK earn a Value grade of A, while WST has been given a Value grade of D.Both MCK and WST are impressive stocks with solid earnings outlooks, but based on these valuation figures, we feel that MCK is the superior value option right now.Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report McKesson Corporation (MCK): Free Stock Analysis Report West Pharmaceutical Services, Inc. (WST): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research