The place I get my gas will only replay the deposit if it's returned within year, the longer you take, the less they pay back (Up to a maximum of 3 years in which case they charge you the cost of a new tank). I only say this as I didn't find this out until the gas was delivered so you'd better contact them and find out if they run a similar scheme.
It depends on the fill pressure.
Some suppliers don't fill to a high pressure, so a 20l bottle doesn't always hold much. For example, BOC fill to 230 bar, some rent free only go up to 137, yet they can be sold in the same size cylinder.