Investors looking for stocks in the Transportation - Services sector might want to consider either Echo Global Logistics (ECHO) or Grupo Aeroportuario del Centro Norte (OMAB). But which of these two stocks offers value investors a better bang for their buck right now? We'll need to 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 Zacks Rank is a proven strategy that targets companies with positive earnings estimate revision trends, while our Style Scores work to grade companies based on specific traits.Echo Global Logistics and Grupo Aeroportuario del Centro Norte are sporting Zacks Ranks of #2 (Buy) and #3 (Hold), respectively, right now. This means that ECHO's earnings estimate revision activity has been more impressive, so investors should feel comfortable with its improving analyst outlook. But this is only part of the picture 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 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.ECHO currently has a forward P/E ratio of 25.97, while OMAB has a forward P/E of 34.29. We also note that ECHO has a PEG ratio of 1.48. 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. OMAB currently has a PEG ratio of 11.43.Another notable valuation metric for ECHO is its P/B ratio of 1.97. 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, OMAB has a P/B of 3.66.Based on these metrics and many more, ECHO holds a Value grade of B, while OMAB has a Value grade of C.ECHO stands above OMAB thanks to its solid earnings outlook, and based on these valuation figures, we also feel that ECHO 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 Echo Global Logistics, Inc. (ECHO): Free Stock Analysis Report Grupo Aeroportuario del Centro Norte S.A.B. de C.V. (OMAB): Free Stock Analysis Report To read this article on Zacks.com click here. Zacks Investment Research