I've used both Aperture & Lightrooom – though obviously my investment in Aperture is higher (having actually purchased a copy), I find it a much nicer product to use with large photosets.

The biggest thing about Lightroom is probably that in Aperture you can do serious stuff with just thumbnails, (organisation, stacking, so on), then hit V to bring up (down?) the viewer. Then hit V to make it go away again. Plus much larger range of "zoom" levels on the thumbnails. Plus I've probably become a lot more fluent with the various keyboard shortcuts there

However, one thing that I love about both products is the non-destructive RAW editing, and I should really give Beta 3 another try.

cheers