The way I'd look at it is the more times you'll need to pack and travel with your bike the more I'd be inclined to use a folder. If I had a few long tours with air travel at each end I'd tend to favour the S&S idea.
Another thing to look at is if you already have a good touring bike adding a folder to your fleet gives you a whole bunch more versatility even when you are at home. It is easy to throw a folder into a car for work or family trip and get some riding in at your destination with minimal hassle.