Investors looking for stocks in the Financial Transaction Services sector might want to consider either Alliance Data Systems (ADS) or Equifax (EFX). But which of these two stocks is more attractive to value investors? We'll need to take a closer look to find out.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 Zacks Rank favors stocks with strong earnings estimate revision trends, and our Style Scores highlight companies with specific traits.Alliance Data Systems and Equifax 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 ADS has an improving earnings outlook. But this is just one piece of the puzzle for value investors.Value investors also try to analyze a wide range of traditional figures and metrics to help determine whether a company is undervalued at its 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.ADS currently has a forward P/E ratio of 4.40, while EFX has a forward P/E of 37.50. We also note that ADS has a PEG ratio of 0.33. This popular figure is similar to the widely-used P/E ratio, but the PEG ratio also considers a company's expected EPS growth rate. EFX currently has a PEG ratio of 2.53.Another notable valuation metric for ADS is its P/B ratio of 1.77. 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, EFX has a P/B of 9.69.These are just a few of the metrics contributing to ADS's Value grade of A and EFX's Value grade of C.ADS 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 ADS is likely the superior value option right now. Zacks' Top Picks to Cash in on Artificial Intelligence In 2021, this world-changing technology is projected to generate $327.5 billion in revenue. Now Shark Tank star and billionaire investor Mark Cuban says AI will create "the world's first trillionaires." Zacks' urgent special report reveals 3 AI picks investors need to know about today.See 3 Artificial Intelligence Stocks With Extreme Upside Potential>>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 Alliance Data Systems Corporation (ADS): Free Stock Analysis Report Equifax, Inc. (EFX): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research