$50 is for the two day pass. $30 for one day.
I guess that warners may be better value in terms of beer/$ but B&T offers alot more in terms of a quirky venue, easy public transport, unique beers (some brewers such as murrays and potters make special beers for the day, as well as the homebrew which is usually decent at worst and fantastic at best), as well as things other than drinking (music, information sessions etc.).
No reason not to go to both