This is a question I get asked fairly frequently, including just today in a comment by fallenpegasus to my last post.
The answer is not QUITE as simple as it appears. Monetarily, I'm pretty sure that Webscriptions (the Electronic Text version of DK that you pay for) gives me more money per copy than the dead tree format. However, the dead tree format is what most people track, including distributors and large book chains like Amazon, Borders, B&N, etc., so an electronic sale doesn't have any effect on their numbers -- which will very likely affect the volume of re-orders they place with distributors and the future volume of orders for NEW books by me.
OBVIOUSLY, therefore, as loyal readers and supporters of my literary ventures, your only option is to buy BOTH! ;)