Well, since I haven't posted on any of the other threads.... 😀
Aperture:
Flexible 'workflow'
Fancy web gallery, book, etc. creation
Generally better organizer, I find it's easier to sort my incoming photos
Multiple monitor support -- very nice, and incredibly useful to LR/Aperture's main job of photo management, so I'm sure the next update to LR will have this
Lightroom:
Seems generally better at developing, as you might expect from Adobe
I love how it provides automatic smart sorting for shots by lens, camera, etc.
I like the B/W conversion options -- seems much easier and more flexible in LR than Aperture
Ultimately, I chose Aperture, but it was an agonizing choice. I hope in the next versions they rip off each other's best features and gain the ability to import each other's libraries, at least to an extent. 😛 I'd also like to see at least Aperture and iPhoto play more nicely together.