40g here as well. If an option involves a size, I'd always go for the bigger one. In fact I'd max out the size depending on the space where the tank is going. If I could fit an 8ft tank in a particular area, that's what I'd prepare for. Big tanks=stable tank in most cases. More maintenance sometimes but you can set up the level of degree how you'd like meaning, Low tech-Very High tech.
Ultimately you have to have a vision of what you want to stock. Fish,plants and your intent for them. Start by knowing how big of a tank can you deal with. Now if one is just beginning the hobby I think 20-40g are good sizes. Alot of newcomers start with 10g. Alot of problems can happen here. There is a smaller margin for error the smaller the tank with the bio load.