for a 40 i would use hob. When you upgrade use the 40 as a sump. ok, i didn't do it that way.
when you look at options it helps to know whether the goal matches the path.
if you want to learn about whether you like the hobby, go simple and easy.
if you want a specific fish or other livestock look at what it needs and decide if you want that much of an investment.
if i wanted to keep a fish that needed a lot of room but i couldn't do that right away i would build a simulation of the environment that it needs and learn how to maintain the eventual system on a smaller scale.
if water quality is the issue i would learn to maintain that level of water quality on a small system with the equipment and processes that are needed.
after i have created the environment, then, I would add the inhabitants.
remember that one way to master this hobby is one step at a time. happily each step is rewarding.