Personally, I don`t like to have a bunch of equipment attached to a QT because it is too hard to disinfect if the need comes along. I prefer to just have a simple tank with lid, dim light, airstone, heater, and a small powerhead. Pretty much what you have in that 20-gallon tank. I think your 70-gallon tank would be too complicated for a QT. Some fish some even need the powerhead and would do best with just the airstone. You definitely don`t want an anorexic fish spending energy swimming against a powerhead. I like to use a couple good-sized pieces of live rock and a few PVC pipes. If there is disease outbreak, remove the live rock, and the PVC pipes that the fish is already familiar with will remain in the tank. Just remember, as soon as the tank has no live rock it also has no means of dealing with ammonia so you need to use a detoxifier.
Do you have space as soon as you walk in the room to your right? You could put it on a stand that has shelves in it for storage...2-in-1 oh yeah!
