We can say something about pros and cons for these products, because they're built on top of different architectures and implemented differently. But instead of just comparing what-can-do or cant-do, I'd suggest doing some "mash-up" of these services. Let's say it's a meta-mashup.
Pipes accepts RSS/Atom feeds as an input, and doesn't have an ability to detect site updates that don't come with feeds. Dapper and Plagger are good at it. So you use Dapper's cool UI to analyze and generate API for any web pages, then consume that generated feeds as an input to Pipes, and create a flow to generate remixed feeds.
Dapper and Pipes are both hosted on the server and hence there's no way to interact with local devices or personally authenticated services, like storing updated feeds to iPod, or notify via MacGrowl or post the updates to Twitter using your account, which Plagger is really good at.
So you can mash up these remix sites to create a fully GUI controlled API creator (Dapper) and Pipes-flow programming (Pipes) with your own publisher/notification engine (Plagger).