I actually think the 40 suggestion is a way to get more used tanks into the market (nobody wants a SA).
I agree, you should buy based on your diving, however it is also prudent to try and understand where your future diving might take you and buy based on that as buying simply for todays diving will ensure you buy, sell, buy, sell as your diving matures (and I say that from experience).
Can you explain what you meant by the above quote as I am missing your point? I don't understand how buying a 40 will increase used cylinders on the market?
I do know from experience here (OZ) most divers with some experience will not generally buy a 19 as they are virtually not used here for anything other than generally recreational diving (usually by those who are buying their first sling cylinder) or perhaps 100% O2. Yes here most tech divers buy slings as a minimum size of 40 or 75 if diving deeper. Its fairly standard in the industry rather than "because everyone does". When I started my tech course the instructor told me not to bother with my 19 so I only use it when recreational diving and usually only to allow newer divers with no sling cylinder some experience with either using it or being able to get to use it as a secondary air source. I would say however if you are very sure that recreational diving (and usually limited to 100' or less) is the only diving you will be doing, then a 19 would do. I know which is easiest to resell here, a 40.