Inside Aperture blog contributor James Duncan Davidson writes on his own blog about Aperture vs Lightroom RAW Conversion:

Does this mean that Aperture is off my system? Heck no. I'm not tossing 31,000+ images at Lightroom anytime soon. Aperture kicks Lightroom's butt at organization and management. And Aperture has done a great job with generating screen-based images for display. But, Lightroom kicks Aperture's butt at rendering RAW for printing at large sizes?at least some images. With others, Aperture holds its own just fine. At least right now with Aperture 1.5.1 and Lightroom Beta 4.1.

Having loaded the Aperture trial on my Mac over the weekend, I'll post my own thoughts soon. And I will be fair to Aperture, honest.