You might be interested in this article I wrote for my website
Here
I've tried both and there are pros and cons for both programs:
Lightroom
Pro:
Speed
Price
Usability
Con:
Restrictive
No dual monitor support
File managment is poor in comparrison to Aperture
GUI needs more work as there are many things that aren't visually easy to follow, Stacks being just one.
Aperture
Pro:
Immensily powerful
No need to re-build web galleries etc when you add a new photo unlike Lightroom
File Mangement stomps all over Lightroom
Totally flexible
Dual montitor support as standard
Con:
Price
Speed - You need a really beefy Mac to get the same performance as Lightroom on an average spec Mac
Steep learning curve
My personal opinion is that at the moment Aperture is the best program out of the two. However I would also say that Lightroom will almost certainly end up being the better program for the simple reason that it has Adobe and legions of photographers behind it that want Lightroom to improve. Apple and Aperture simply can't compete at the same pace IMHO.