I bought Aperture because I had tried the first public beta of Light room and hadn't liked it much. Aperture has a very nice interface in many ways, but even on a new iMac with an enhanced graphics board and additional RAM, it ran slowly and I found it froze a bit more frequently than I would have liked. When the final version of Lightroom was released, I downloaded the 30-day free trial version and was converted in a half hour or so. I personally think it is superior to Aperture in almost every way. Maybe it is years of using other Adobe programs (Illustrator, InDesign, etc.), but I found the interface and the way it organized things to be more intuitive. It is complex, and does take some time to learn, but I like it much better in the end. It is much faster to load, and in my experience, much more stable. And perhaps most importantly, the image processing functions are more sophisticated. If you use Photoshop, too, the integration is really excellent.
Having had the experience of using both programs for several months, my choice is definitely Lightroom. But Aperture has many nice features as well, and I think issues of personal preference are involved. I can't remember if Aperture has a downloadable demo version or not, but I highly recommend you download Lightroom and try it for yourself. One more thing: lightroom is offering a promotion until the end of April, I think, which allows you to buy Lightrom for $199. That's a bargain.