you really not saving much money between 20g and 40g. the start up will cost just as much. IMO
I disagree with this. Those of us who have been in the hobby for a long time have the fortune of having spare parts and equipment laying around.
Starting fresh, even if you buy secondhand items online, there will be a big difference between the cost of putting together a 20 vs a 40 gallon tank. Just a 20 long tank itself is less than half the price of a 40 gallon. A 20 long will also fit on top of a lot of sturdy shelves, dressers or countertops the average person will have, while the length and width of a 40 will almost necessitate buying a specific stand. The difference in cost for appropriate pumps to move water will be very noticeable.
For a young, new hobbyist, those cumulative savings can make a huge difference in stretching a budget to allow for good equipment, fish, corals, etc.