I am a member of both programs. As you know SPG is free and is run by Starwood. Privilege is run by an outside marketing company and annual fee is around $275, however you get 2 cards, and can nominate anyone for the second card. So if you find someone else who wants to join you could split the cost. Their main selling point is dining discounts, but you also get vouchers for discounts, upgrades etc. There is also a 10% off Best Available Rate, but often this is still not as good as "package" rates on SPG, so don't expect to save 10% on every stay. This link gives information
:: Starwood Privilege Asia Pacific ::
Whether the Privilege program is worth the cost really depends on your travel habits. I worked out that next year the worst I could do was break even, so I decided to try it, but I spend a lot of nights at participating Starwood hotels. You really need to look very carefully at the benefits and T&Cs, then work out if the cost is value for you.