Major differences are :
platinum you get a snack (bev and food ) everday plus the BLD, premium is only BLD
plat gives you more tours like dolphins and divequest
plat allows you to eat at V&A premium doesn't
plat gives you fireworks cruise or illuminations cruise filled with other plat guests, no cruise on premium
plat gives you a spa treatment
plat gives you trip planning services through the concierge desk (some resorts may also bump your room to concierge level)
may also have fewer restriction on golf tee times but since I don't golf I'm not sure
In room child care instead of just kids clubs
The main reason we jumped from prem (which we usually use) to plat was b/c we wanted to do the epcot tours. The cost for us to add plat was $500 which is right around what we would have had to pay extra for those 2 tours, plus we'll get an illuminations cruise and a massage out of it for free.