after rereading your post -
here is what i would do...
buy the cylinder that make sense now for you but schedule and take primer (it is not a pass/fail class nor is it in any way a requirement for other classes)
in the class you may find that the tank is right or completly wrong for you, it all depends but the main thing is what you take away from the class, not what you bring into it.
I would speak to the instructor once you register about it though.
here is what i would do...
buy the cylinder that make sense now for you but schedule and take primer (it is not a pass/fail class nor is it in any way a requirement for other classes)
in the class you may find that the tank is right or completly wrong for you, it all depends but the main thing is what you take away from the class, not what you bring into it.
I would speak to the instructor once you register about it though.