I don't know where u live, but tanks are harder to control than tubs. They lose heat easily and you'll have humidity drops and spikes. I used to insulate mine. Tubs also provide the ability to hide in peace and quiet that tanks cannot provide when they are on display. Look into a rack.

If you really want a tank then get a 20. 40 would be too big and could cause some issues even if its cluttered. Plus, clutter makes cleaning harder and improper cleaning leads to a sick/unhealthy animal. You can find them pretty cheap. You can use aquarium backgrounds. I did when I first started. When the 20 starts to get small u can try a 40, but by then i bet you'll have more and will be looking into racks