Basically SilkTest is a QA regression tool SilkPerformer is a system performance tool.
When using SilkTest you can test functionality, layout and content depending on how detailed you want to be.
SilkPerformer on the other hand only cares that it can get from point "A" to point "B" functionally in a product. For the most part it does not validate the pages it go through. Its whole purpose is to provide load to the servers.
They also operate on different languages.
But in the end doesn't it all come down to BEER? Beer is the ultimate answer to all questions in the universe so yes the answer to your question is BEER.