Using an age 28 equivalency as a benchmark Griffey leads Trout in home runs (350 to 302), RBIs (1,018 to 798), and base hits (1,569 to 1,380). Trout leads Griffey in career batting average through age 28 (.304 to .300) and walks (838 to 656).
The above was from last spring and Trout on his current pace will undoubtedly pass him without catastrophic injury or decline. I'm a fan of using WAR and its derivatives, but it's not as easy pick as some may think.