Zack Greinke is (I believe) a better pitcher than Matt Garza. But Garza's under team control for an extra year, almost assuredly earning less money each season. That monetary savings can be spent on other helpful pieces, bringing the two players closer in value.
Below is a little calculator you can use to see if talent or cost wins the day. By changing the numbers in the green boxes, you can see how each player's contract provides savings relative to acquiring their production on the free agent market -- the TOTAL SAVINGS line at the bottom.
Hit the "click to edit" button to start. Go crazy. This is a model, giving you the opportunity to see how your views affect the comparison. Feel free to share your inputs, outputs, and line of thought in the comments.
Quick note on FA compensation: There's a good chance compensation will be gone by the time these guys are free agents. However, feel free to add it in. Type A status is worth about $6M on average, Type B $3M.