Rob Boyer's All Things Photography blog includes Aperture tips and has also ventured into the dangerous waters of direct Aperture versus Lightroom comparisons. While overall Rob's about as fair and balanced in advocating Aperture as I am in preferring Lightroom, some judgements fall in Lightroom's favour. For instance, see his comparison of Aperture and Lightroom keywording:

[In Aperture] you can do crazy stuff like running scripts that smash the entire hierarchy into each of your images based on the specific keywords but that sort of defeats the purpose. Lightroom on the other hand will export the entire hierarchy for each specific keyword, but wait there's more, for each and every keyword Lightroom let's [sic] you specify whether or not to include the parent keywords and… whether to export it at all.

…Score a big win for Adobe Lightroom 2 versus Apple Aperture 2 when it comes to keyword functionality.

While I think his conclusion is right in this case, it shows a couple of dangers of such comparisons.

One is that it's easy to have misconceptions about how people actually use “the other” program's better features. Lightroom's keywording is so very flexible that a lot of people are led to abuse its power, exploiting the feature to add keywords which do not describe the image. So I've heard plenty of cases of people including workflow-related keywords like “not done”, “developed”, “final”, or including stock agency names as not-to-export keywords. Worse still is when they're encouraged to exploit Lightroom's keywording in this way without always understanding that it is a workaround for proper database and cataloguing features. While the keyword-abusive approach can certainly be made to work, sooner or later someone forgets to tick that do-not-export box, there's a need to exchange metadata with another app, or the export flag is misread by a bug in a new version's upgrade process. Suddenly you've got private keywords, custom metadata, getting into files exported from the system, and your filters and smart collections are picking up items with “workflow keywords” when they're supposed to target proper, descriptive keywords. Isn't any workaround always destined to be a dead end?

Rob's post is also a great example of the dangers of considering a single feature in isolation, because while Lightroom's keywording may well be better than Aperture's, the feature's flexibility allows Adobe to get away with failing - unlike Aperture - to include a proper custom field feature, which is where custom metadata really belongs.