Ah bless, you and your questions!
Propane all the way. It's EXACTLY the same as 'green gas'. You can buy adaptors that attach to the small Coleman propane bottles that come in twin packs. These bottles hold 465g if propane (which is 1.17 litres, so as to avoid confusion with the usual method of quoting ml on airsoft gas cans) and cost between £10-20 for 2. Massively cheaper than rip-off airsoft gas .
The problem is... these have become very difficult to source in the UK. But I have a solution on the way! You can buy adaptor hoses that link to the big bottles of propane ( I mean anything from the 3.9kg to 47kg big cylinders) to which the propane airsoft adaptor will fit. The 3.9kg bottle is only £15 to refill each time - you do the math!
When I have my adaptor, I'm thinking of saying anyone who wants to use it can have as many mag refills as they want in a day for £3. That way anyone on a game day can get cheap gas, and I will be covering the cost of refilling again when it's empty.
Otherwise, go get yourself an 'Airsoft Innovations' adaptor - well worth the money. They even come with a spare tip and some silicone oil (only difference between airsoft propane and the normal stuff is the addition of silicone oil)
Nuff said.