There's a
sticky over at the Digital Photography forum regarding Aperture vs. LightRoom.
It seems rather evident that you cannot mention aperture with hearing about LightRoom.
I'd suggest OP, that you download aperture (and LR) as both offer trial downloads.
I've used Aperture since day 1, but I've just recently switched over to LR. The reasons for my jumping ship have to do with the editing tools offered in LR over aperture. Aperture has a great suite of tools to organize and categorize the images and its integration with OSX and apple apps is also very nice. I found it very lacking the tools edit my images and its organization structure a bit rigid and forces me into a hierarchical layout that over time, got a bit cumbersome.
Proponents will tell you that that's why apple built in the plug-in architecture - to allow more advanced editing. There's one small problem with that, plug-ins use destructive editing, where as LR provides more tools that are non-destructive.
Finally, I've opted for LR as many of the photographers, I'm concerned that apple's commitment to aperture, especially given the market share.
Take a look at this
adobe blog which has some market share numbers Even if you take the numbers with a grain of salt because they came from adobe, the shear difference is stark.