Inside Aperture has been running a side-by-side comparison of Aperture and Lightroom with a photojournalist out in the field. Though I'm afraid that, as with most things, it seems like each has some things they do better. Aperture's more intuitive, Lightroom's got a more powerful web-export model, Aperture's got far better batch operations, Lightroom's got more powerful greyscale and color-curve features, etc.
For me, I found Aperture works better for my flow, partly because I didn't like the difference between 'Develop' and 'Library' modules. I know that it's the fact that Aperture lets you tweak a photo from ANY part of the flow that does give it a slight speed hit, but dang it, sometimes I really /want/ to tweak the picture from right in assembling a book or a web sheet or whatever. That said, Lightroom is pretty fast, and though it's not quite as intuitive, it's got the Adobe Power behind it, as it were. And being able to use NoiseNinja instead of Apple's (unquestionably) more mediocre noise reduction... well, that shouldn't be underestimated.
I think it comes down in the end to what you need or want out of a program, and so what works/feels best for your personal taste and work-style.