Welcome to R2R! I would also recommend the 40L. The closer the size is to what you think you will want long term, in your house when you get it, the more of the equipment you buy will be useable on that tank. Most equipment designed for nano=sized tanks don't really end up being that useful when you upsize. Also the 40L will keep you happy longer because of the better selection of compatible fish and the room for corals/aquascaping. It might even turn out to be all you ever want. 40 gallon tanks are kind of a sweet spot in the hobby.