Here’s a “Aperture v.s. Lightroom Comparison”:http://blog.duncandavidson.com/2006/09/aperture_vs_lig.html.
One thing he didn’t mention is what kind of hardware he runs these programs on. Aperture on a Powerbook G4 is pretty painful. But then again, most apps are. From the sound of it, he must be using at least some kind of G5.
Also, it’s interesting that he doesn’t find the single monolithic library approach that limiting. Maybe it’s just not a problem when you can afford big disks.