Investors interested in Medical Services stocks are likely familiar with ModivCare (MODV) and Charles River Laboratories (CRL). But which of these two stocks presents investors with the better value opportunity right now? Let's take a closer look.Everyone has their own methods for finding great value opportunities, but our model includes pairing an impressive grade in the Value category of our Style Scores system with a strong Zacks Rank. The proven Zacks Rank puts an emphasis on earnings estimates and estimate revisions, while our Style Scores work to identify stocks with specific traits.ModivCare and Charles River Laboratories are sporting Zacks Ranks of #2 (Buy) and #3 (Hold), respectively, right now. The Zacks Rank favors stocks that have recently seen positive revisions to their earnings estimates, so investors should rest assured that MODV has an improving earnings outlook. But this is just one piece of the puzzle for value investors.Value investors are also interested in a number of tried-and-true valuation metrics that help show when a company is undervalued at its current share price levels.The Style Score Value grade factors in a variety of key fundamental metrics, including the popular P/E ratio, P/S ratio, earnings yield, cash flow per share, and a number of other key stats that are commonly used by value investors.MODV currently has a forward P/E ratio of 27.99, while CRL has a forward P/E of 39.74. We also note that MODV has a PEG ratio of 2.80. This metric is used similarly to the famous P/E ratio, but the PEG ratio also takes into account the stock's expected earnings growth rate. CRL currently has a PEG ratio of 2.84.Another notable valuation metric for MODV is its P/B ratio of 6.15. The P/B ratio pits a stock's market value against its book value, which is defined as total assets minus total liabilities. For comparison, CRL has a P/B of 8.83.These metrics, and several others, help MODV earn a Value grade of B, while CRL has been given a Value grade of C.MODV has seen stronger estimate revision activity and sports more attractive valuation metrics than CRL, so it seems like value investors will conclude that MODV is the superior option right now. Breakout Biotech Stocks with Triple-Digit Profit Potential The biotech sector is projected to surge beyond $775 billion by 2024 as scientists develop treatments for thousands of diseases. They’re also finding ways to edit the human genome to literally erase our vulnerability to these diseases. Zacks has just released Century of Biology: 7 Biotech Stocks to Buy Right Now to help investors profit from 7 stocks poised for outperformance. Our recent biotech recommendations have produced gains of +50%, +83% and +164% in as little as 2 months. The stocks in this report could perform even better.See these 7 breakthrough stocks 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 ModivCare Inc. (MODV): Free Stock Analysis Report Charles River Laboratories International, Inc. (CRL): Free Stock Analysis Report To read this article on Zacks.com click here.