I prefer the "20" pounder. Its actually only 16.1 pounds, but people call it a 20 pound cylinder. The same one we all use on our bbq grills. The main reasons I use this over the smaller ones? They last a LONG time, and you can find them ANYWHERE without having to find a place that will fill that fancy little cylinder that you specially order. Most gas stations, Wal-marts, Home improvement stores etc around here swap them out for a decent rate, which means I can get propane 24-7 if need be. They arent that much bigger in terms of space either...
True, plus some propane dealers have a minimum charge. I can remember dealers who charged the same price to fill a half empty 20 pounder as they charged for empty one. And this was for a refill, not an exchange. If you hit these minimum sale dealers for a refill, the economics of filling the smaller 5 or 10 pounders is moot. If you aren't pushed for storage space, the 20 pounder can save you a few bucks at times.