How would one go about building such a product though? Comparing hardware features (say iPhone with Samsung for example) is a lot easier than software, isn't it? SaaS products can release features every week, it would be quite hard to keep up.

It would be cool if there is a JSON standard to publish features (and pricing) for SaaS apps - something like what AWS does for its products. Defining such a standard itself would be tricky, but it is worth trying. It might make it a bit easier to compare various project management apps, for example.