Lots will be said about these two RAW-workflow tools (and already has been said), but with both applications having trial versions available, you really need to try them for yourself. I found the Ars Technica review especially interesting as it provides sample images from Lightroom, Aperture, Bibble and CaptureOne so you can see the differences in resolution, color, noise, etc.
Lightroom: “Innovates in its own way by providing a mother lode of intuitive adjustments, many ways to navigate and tag a huge library of images and it does it with speed to spare.” (via ArsTechnica Review)
Lightroom: “Adobe’s next generation photo software is a full-featured, all-in-one imaging solution for serious shooters.” (via PopPhoto.com)
Aperture: “(Aperture 1.5.2) brings the program even closer to fulfilling its promise as the start-to-finish, import-to-output hub of professional and advanced photographic workflow. Serious digital photographers should give it a look.” (via MacWorld Review)
Aperture: “The program’s non-modal interface; true real-time, non-destructive raw workflow; and excellent organizational tools put it head and shoulders above the competition in terms of easy, efficient workflow management and post-production.” (via CreativePro)
Also, enjoy these resources for both apps:
Lightroom Learning Center (NAPP)
Aperture Resource Center