Even if you have a Mac, it has to be a fast, new Mac with a big screen to really take advantage of Aperture. I tried it on my 15" Powerbook and it worked but was quite frustrating. Lightroom, on the other hand, has worked very well indeed on my wee laptop. I think of Aperture as being the tool for the big budget photographers and Lightroom for us more humble folk! Tongue in cheek though, I may have picked Lightroom anyway...
Woops, forgot my main point - which is that there are many more "humble types" out there than the big budget folk, hence I would expect Lightroom to do better by ten or twenty to one.
Don.