This is just a an observation of my own work habits as I use both of these programs quite a bit.

I have found that if my workflow is leaning towards online presentation, I tend to use Aperture2 more than LR2. My cameras are set to sRGB and Aperture2 always generates images that are very pleasing onscreen. Printing from Aperture2 is quite the torrid affair.

However, if I intend to print on my B9180, LR2 beats the pants off of Aperture2. Culling and adjusting images for print takes little or no time and I can print with my watermarks very easily. Color is pleasing to the eye on the final output despite the fact that LR2 does not yet offer soft-proofing like Aperture2.