The only reason to stay with 2007 is if there is an app that you have to have that's not available in 2008. That will remain true even after the official 2008 is released. As a new user with no 'favorite apps', I'd say go ahead with 2008beta, it should be the easiest to transition to 2008 standard unless you can put it down for the next few weeks waiting for the official release (not likely).