The 30 days free / included sub start the day you register the PS3 copy - if you register it to the same SE Service account you've played the PS4 beta Stage 2 on you'll have all your characters and progress there. On Friday you can upgrade to PS4 and continue to play there during Early access. So effectivelly if you'd buy the and activate the PS3 version today you'll "lose" only 2 days anyway.