A story in the Wall Street Journal a few days ago said that Apple may be selling as much as 20% of non-corporate computers, with very strong growth since they went to Intel. Of course, corporate sales are huge, so overall, Mac sales are still a small fraction of overall sales, but the fraction is growing quickly. I saw another number recently that said about 10% of all computer sales are now Mac, but that story cited the Wall Street Journal story, and since the WSJ story didn't say that, I'm not sure about the number.
In any case, Aperture sells far fewer copies than Lightroom, and probably will continue to do so. Part of it is simply a bad attitude on the part of Apple, and the other part is that Lightroom is available for both PC and Mac, and Aperture isn't.
By bad attitude, I mean that Leica M8 users, for example, were told bluntly by Apple reps that Aperture would not support the M8 because it wasn't a significant camera. There went several thousand pros and semi-pros. Apple's tendency to drag its feet as regards camera updates really hurts.
The fact that Lightroom is dual platform means that a Lightroom user can move seamlessly between Mac/PC systems, without having to relearn. That can be important to professionals and art directors.
Lightroom's relationship with Adobe is, of course, critical.
In terms of the actual app, I looked at both, and Aperture has some strong points. But Adobe updates on a quicker cycle because, I suspect, Lightroom is more important to Adobe than Aperture is to Apple. Therefore, I think, Lightroom will pick up and improve vis-a-vis Aperture more quickly than Aperture will adapt vis-a-vis Lightroom...if you see what I mean.
In any case, I went with Lightroom, not because I liked the actual app better, but because as a database, this is a pretty critical application -- changing horses in midstream would be a major problem. And I think Adobe is more dedicated to the software than Apple is.
JC