Well, if you can afford gold or diamond (compared to silver and platinum respectively) than they would appear to be the better deal. However, some people can't shell out the 500 big ones for diamond but still want a lot of freedom on the site, and thus choose platinum.
The same goes for gold and silver; of course gold is cheaper but its paid at once, where as silver is paid monthly, and some people may feel more comfortable with it; say, a person who wishes to try out membership features but is not sure whether he wants to pay for a year of it.
ok, so it barrels down to how many extra privileges you want on the site (silver/gold or platinum/diamond), and then to how much you're willing to spend. Spend more at once? get a better deal. Makes sense to me, and I hope to you as well.
What's the point of getting silver if you can get gold at a lower price because you pay $4.95 for silver per month and pay $29.95 for gold per year.Also what's the point of getting Platinum for a year when you can get diamond for a lifetime. I just don't get it. That's why I'm staying basic until I determine which one I want.