Will Synergy Advance be free for users who have a Synergy license?
Synergy Advance is a brand new product written from scratch, so upgrade licenses will not be available. Synergy will continue to be marketed as a separate product. Synergy will continue to work when run on Tiger. Although upgrade licenses will not be available, Wincent does offer discounts to those who already hold Synergy licenses; the discounted licenses will be 7€ each.
Should I buy a Synergy license or a Synergy Advance license?
Synergy is a mature product that runs on Mac OS X Jaguar, Panther and Tiger. If you decide to purchase it it will serve you well.
Synergy Advance is still in the early stages of its roll out so it does not yet feature all the features of Synergy (although it does feature some things that Synergy does not and never will have because of its architecture). By the time the roll out is complete Synergy Advance will do everything that Synergy can do and much more (see the road map).
Wincent offers discounts to people who hold a Synergy license, so the recommendation is that you try out both products (free 7-day trials are available) and make a decision based on your trial.