You can check out all of Miro’s perceived advantages here, but to sum them up: Miro is open-sourced, DRM-free, friendly to all content creators, connected to all the popular video sharing sites like YouTube and blip.tv, high definition, full of content, and BitTorrent-enabled. Joost, on the other hand, is proprietary, exclusive towards content creators, DRM-protected, closed to video sharing sites, lacking in content, lower quality, and entirely streaming video.
I’ve tried both Miro and Joost, and I like them both but for different reasons. Miro functions more like iTunes and is a good way to download batches of interesting videos from the internet regardless of whether they are professionally-produced or user-generated. Content must be downloaded via HTTP or BitTorrent, not streamed (although Miro can convert streams into downloads from sites like YouTube, blip.tv, and DailyMotion). You can download particular shows or just tell it to give you videos from particular categories (comedy, news, technology, etc.). Since videos must be downloaded, playback is not instant; but the videos load pretty quickly so it’s not a huge drawback.
Joost, at least on the surface, is better for viewing professionally-produced content instantly. Since Miro claims to match Joost’s number of commercial channels, however, this may be due simply to how Joost focuses exclusively on professionally-produced content. When you use Miro, you must wade through content not produced by major media outlets to find regular TV shows. And then when you find them, you’ll have to wait until many videos load via BitTorrent. Download speeds will then vary depending on how many people have the seeded the BitTorrent videos.
Miro will soon be available for co-branding so that content creators can create their own versions of the player with pre-configured channels filled with their own content.