I haven't used aperture but I stumbled upon this the other day:
http://www.travelphotographers.net/article.php/aperture-vs-lightroom
I surmise from that that if you have aperture, you probably won't need lightroom. But if you haven't purchased either, lightroom might be the better bet for you. (Kind of like minor upgrades to camera bodies or lenses or speedlights, not worth spending the money to upgrade, but if you are going to start from scratch your better with the new version)
I think Adobe still does a free trial of lightroom, they used to. Check it out and see what you think?
EDIT:
Here it is;
https://www.adobe.com/cfusion/tdrc/index.cfm?product=photoshop_lightroom&promoid&promoid=DTEML
Download it and give it a try. Hey it's free why not? Let us know what you think, I'd be curious too how it stacks up with Aperture. Personally I love lightroom, but I've never tried Aperture!