Gold gives you some perks like free and closer parking, early entrance on DB and The Beast, discounts at stores, and some other things. Platinum gives you all of the Gold Perks plus entrance to any Cedar Fair property like CedarPoint, Kings Dominion, etc included with your pass.
If you are planning on going to more than Kings Island then it's worth getting a Platinum pass. I had one this year and went to Cedar Point 4 days and was well worth the money to get the Platinum.
Hope this helps