I used Aperture in the early stages of it's development and found it a bit wanting, It may have improved over the years. ( Also the initial price I thought was way to high, but it's a bit more sensible now).
I've been with Lightroom since the early Beta days so may be a bit biased.
I suggest that you have a look at some of the video's on Lightroom 3 on the Adobe web site. That'll give you a taste as to what it can do. Then I'd download the trial version, and see if it has features that Aperture doesn't and you find useful. I suggest looking at the videos first as you only have a 30 day trial, so seeing how it works saves you some time on the learning curve.