Rick LePage writes in his comprehensive comparison of Lightroom and Aperture, "While both Aperture and Lightroom perform the role of image manager and photo editor admirably, each program has plenty of unique features that set it apart. To determine which program is best for you, you’ll need to assess your work style and then choose the one whose features best support that process. I’ll lead you through the most significant differences and help you decide which will benefit you most."
If you're a Windows user, I think you'll appreciate Rick's breakdown of Lightroom. For those on the Mac, the side-by-side comparison of features is truly helpful. Either way, Rick LePage's Aperture vs. Lightroom: The new digital darkroom is a must read for those considering either or both of these applications.