I am both a Mac and PC user. Aperture is more like iPhoto, and in some ways much more intuitive, so I can understand why you wouldnt like to learn new software. But it might depends on your workflow, because to be honest, despite Aperture's intuitive nature, I found my workflow of edits made much more sense with LR. I dont think anyone who choses to use Aperture has made a mistake, and you're crazy to use Aperture. Your friend sounds a bit like an "adobe-head", just as people who say similar things about Aperture I consider "mac-heads". Both are excellent pieces of software, both with plusses and minuses. (though if you read the individual communities of either youd think that the "other" one barely loads, from some of their comments).
A couple of things that you will get with Lightroom over Aperture: Larger userbase, more plugins. This is a fact when a software supports multiple OS. More and more plugins is a key component to a longevity of a software like this, in my opinion.
I think some of the lighting and highlights work better in LR than Aperture, but that could just be my imagination.
In fairness to Aperture, though, some of the features in lightroom have been in Aperture for some time. However, now that LR 4 has many of Aperture's features, if I were to choose now, I would have NO problems choosing LR. When I did choose about a year ago, I had a hard time making a call. I feel like I made the right one. Adobe HAS been playing some catch up, but I think they have.
Personally, I prefer a software that is cross platform. For example, if I had a ton of files to edit, I could imagine myself purchasing (or renting) a high end PC for just image processing, and if using LR, id have that option. With Aperture, I'd be limitted to my Mac. As a user who uses both OS's, I appreciate software that works on both. (BTW, I also blast PC only software).