O'Reilly's Lightroom vs Aperture has some interesting articles and even more interesting threads such as this discussion of Aperture's flexibility vs Lightroom's modularity. While I prefer a palette-based interface, as in Aperture, I don't think Lightroom's modular approach slows me down, and one commenter put it really well:

I'm still confused as to why modularity is a problem. To me, Aperture seems to be great for someone with a short attention span. Let me explain…

In LR, I use the Library to determine which images from the last batch of photos I wish to keep. I keyword them, tag them and rate them. I then switch to the LR Develop mode and begin tweaking my images. I start with the highest rated and work my way down. Therefore, if some of the higher rated images capture the scene better, I don't have to mess with the lesser images which may require more correction. Seems fine to me.

Now Aperture seems strange doing the same thing. I find myself sorting my images, adding keywords and then thinking hmm… what happens if I make this adjustment, or that adjustment. Meanwhile, I've stopped sorting images and started playing with them. LR eliminates this temptation and forces me to make critical decisions where they need to be made.