I believe I read some place that you will have to uninstall the beta SP1 when the finished one is released. Why bother unless you have a reliable source. Who knows what you get from the torrent sites.
The official beta and rc versions will be available, which you will most likely need to uninstall before updating to final version, just as it has been with XP and Vista service packs. The purpose of pre-release versions is to collect users' feedback.