I own Apple stock but no Adobe stock - is that enough of a reason?
I tried LR when it first came out, which was well after Aperture was released at the time I saw no real advantage to it over Aperture. At this point, both packages have matured a lot and without using Lightroom extensively it would be tough to say what advantages either one has. I do know that I much prefer Apple's default RAW conversion settings over Adobe's and I find it's easier to get accurate color prints using Aperture vs CS3 as well.
Over the long haul I believe Apple will continue to add photoshop type capabilities to Aperture as fast as they can without getting Adobe too riled up. If you look at the history of final Cut Pro you'll see the same type of evolution.
Bottom line is I don't think you can go too far wrong with either. I've put the time in with Aperture so short of some huge advantage in LR I won't be switching.