Having both programs, I wouldn't underestimate the difficulty of a Lightroom user comparing it with Aperture, and an Aperture user coming at it from the other direction, but that's exactly what Michael Clark on O'Reilly's Lightroom Blog and Micah Walter at O'Reilly's Aperture Blog are attempting (though Micah does seem to be writing mainly about Aperture). Already there are plenty of moments when I've thought “oh yes it does” at some comment about a feature's absence.

Coming at it from Lightroom too, I couldn't agree more with what Michael says on stacks :

Now in terms of Stacking, I was never one who stacked selects on my light table so this function seems a little strange to me but I know of a few Aperture users who swear by it. After playing with stacking in both Lightroom and Aperture, I have to say that it is much better done in Aperture because of the visual separation between stacks and the control you have as to how the stacks are made. Lightroom has similar controls to adjust and automatically form stacks but with the images all lined up right next to each other it isn't visually easy to delineate where one stack starts and another ends. For my workflow, I don't use Stacking so I'm not too worried about it. I tend to sequester my images by their star ranking and it works fine for me.

Functionally, stacks are identical in the two programs - it's simply the way they are represented visually in Aperture that makes the difference.

He's making two points, however, and and I agree with his second one every bit as much. This is regardless of who does it better - I've never really liked stacks either. Apart from the data not being portable between DAM systems, stacks have always seemed the best way to hide and lose track of that great shot that you notice later - sometimes many years later. You don't have to use them.