There is no doubt that refilling is a better value if the logistics work for you.
The empty weight of the tank is stamped into the handle, so it's a simple matter to figure out how much propane is in the tank by weighing it. I weighed a few exchanged tanks a while back out of curiosity, and I was getting 15-17# of propane. The labels say 15#.
When you refill, you pay by the pound, not by the tank, so if you get 5 more pounds, you pay for 5 more pounds.