If you want to be safe and use the runway you just departed as your emergency field, Vx. Plot the glide path for each, including a safe 180. Best glide speed will always favor proximity over altitude. (No Obstacles, Right?).
There you go, that's what I am trying to discuss in this thread.
And I do disagree with what you say for the following reasons (which has been explained previously by myself and also others in this thread):
1. Vy should still be able to get you enough altitude to make it back even though you are further away (unless you have certain atmospheric conditions like
strong tailwind on a short runway)
2. With Vx, you might not even reach the safe turn back altitude by the time you would with Vy, and therefore climbing with Vx might force you to have to land straight ahead instead of being able to safely make the turn
3. Vx makes you more susceptible to an inadvertent stall
4. Vy leaves you more both kinetic and potential energy
5. If you lose your engine at Vx, the altitude lost by increasing your speed to best glide speed will most likely equate to an altitude loss equal or greater to the advantage of being closer to the rwy vs Vy.
EDIT: and to answer your question: yes, we are assuming no obstacles for this discussion.