Personally I much prefer Lightroom, though I suspect you'll find this argument goes on as long as the Mac vs PC or Canon vs Nikon debates.

Lightroom is more expensive but has far more features, and (surprisingly) seems easier to use, but that may just be because I'm more used to it. The main advantage of Aperture is that it integrates with your other iLife apps, and has (I believe) built-in print ordering options, much like iPhoto does.

Aperture doesn't have as many image adjustment tools as Lightroom, like split-toning (where you colour the highlights and shadows different tints), and graduated filter, and the noise reduction in the latest version of Lightroom is mind-blowingly good. Neither of them supports HDR images as far as I know, but Lightroom integrates more tightly with Photoshop (as you'd expect) which is where you may be doing most of your HDR work.

You can download trial versions of both so it's worth giving them a try and seeing what you prefer working with.

(One other consideration - Aperture requires an Intel processor, not sure what you have but if it's G5 or earlier then you can't use Aperture)