Investors interested in stocks from the Chemical - Diversified sector have probably already heard of LyondellBasell (LYB) and Air Products and Chemicals (APD). 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.LyondellBasell and Air Products and Chemicals are sporting Zacks Ranks of #1 (Strong Buy) and #2 (Buy), respectively, right now. This means that LYB's earnings estimate revision activity has been more impressive, so investors should feel comfortable with its improving analyst outlook. But this is just one piece of the puzzle for value investors.Value investors also tend to look at a number of traditional, tried-and-true figures to help them find stocks that they believe are undervalued at their current share price levels.Our Value category grades stocks based on a number of key metrics, including the tried-and-true P/E ratio, the P/S ratio, earnings yield, and cash flow per share, as well as a variety of other fundamentals that value investors frequently use.LYB currently has a forward P/E ratio of 5.66, while APD has a forward P/E of 31.36. We also note that LYB has a PEG ratio of 0.71. 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. APD currently has a PEG ratio of 2.77.Another notable valuation metric for LYB is its P/B ratio of 3.65. The P/B is a method of comparing a stock's market value to its book value, which is defined as total assets minus total liabilities. By comparison, APD has a P/B of 4.81.These are just a few of the metrics contributing to LYB's Value grade of B and APD's Value grade of D.LYB is currently sporting an improving earnings outlook, which makes it stick out in our Zacks Rank model. And, based on the above valuation metrics, we feel that LYB is likely the superior value option right now. 5 Stocks Set to Double Each was hand-picked by a Zacks expert as the #1 favorite stock to gain +100% or more in 2021. Each comes from a different sector and has unique qualities and catalysts that could fuel exceptional growth. Most of the stocks in this report are flying under Wall Street radar, which provides a great opportunity to get in on the ground floor. Today, See These 5 Potential Home Runs >>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 LyondellBasell Industries N.V. (LYB): Free Stock Analysis Report Air Products and Chemicals, Inc. (APD): Free Stock Analysis Report To read this article on Zacks.com click here.