Investors with an interest in Medical - Drugs stocks have likely encountered both United Therapeutics (UTHR) and Neurocrine Biosciences (NBIX). 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.United Therapeutics has a Zacks Rank of #1 (Strong Buy), while Neurocrine Biosciences has a Zacks Rank of #2 (Buy) right now. This means that UTHR'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 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.UTHR currently has a forward P/E ratio of 8.81, while NBIX has a forward P/E of 745.79. We also note that UTHR has a PEG ratio of 29.37. 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. NBIX currently has a PEG ratio of 35.51.Another notable valuation metric for UTHR is its P/B ratio of 2.12. 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, NBIX has a P/B of 27.47.These metrics, and several others, help UTHR earn a Value grade of B, while NBIX has been given a Value grade of F.UTHR stands above NBIX thanks to its solid earnings outlook, and based on these valuation figures, we also feel that UTHR 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 United Therapeutics Corporation (UTHR): Free Stock Analysis Report Neurocrine Biosciences, Inc. (NBIX): Free Stock Analysis Report To read this article on Zacks.com click here.