You can download trials of both, which I highly recommend.
I wanted to go Aperture, but in the end went LR. The reasons for me were:
1) Aperture seems like a side business for Apple. LR is at the core of one of Adobe's businesses. Apple has a habit of suddenly killing off products not at it's core.
2) Aperture RAW updates always seem behind LR.
3) Lightroom is pc AND mac. Love my macs (I have about 5 in my house!), but nothing is forever. If I ever went back to PC, I can port my LR database over. There's no choice with Aperture.
That being said, the biggest differences right now are workflow: With Aperture you have an image, and can do anything with it. In Lightroom there are 5 modules, and the two most important have some tools divided between them. For instance, you cannot crop or straighten in the "Library" module. You have to go into the develop module. It's just a click, but it's a little inelegant. Some folks hate LR because of it, but all the REST of the benefits outweigh that issue, IMO.
One other side note (personal issue) -- I find the Aperture interface fonts to be too small.
Last note: From what I've seen, there are far more third party sites with tips/helps, etc. for LR than with Aperture. One you can Google for is Lightroom Killer Tips, but there are many others, too.
Try them both -- you've got nothing to lose.