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.