Platinum is definitely worth it for 50/year.
Diamond adds videos and chess mentor, which are good, but you could improve a lot with only the tractics trainer, computer analysis, and game explorer that come with Platinum.
My suggestion, if you want to improve at chess and $50/year isn't much to you, it's definitely worth getting platinum membership. Then, try diamond for a month, check out the videos and chess mentor to see if you like them enough to make it worth an additional $50/year.
I am thinking about becoming a paying member in order to get access to more of the learning tools. I wanted to know from other people who use these if they feel they are worth the price, and if they actually helped you to improve your game.