Is my timer must be on Fixed Update or on Update

Hello
If I put this code on the Update() theoretically the gameobject should spawn at different time dependings of the framerate, but if it is in the Fixed Update() it should spawn at the same time for the guys who have 40fps and for the another one who have 200 ?

if (0.0 >= nextWall)
{
    nextWall = 0.3f;
    GameObject collider;
    collider = Instantiate(Enemy, Rb.position + Rb.transform.forward * distance, Rb.rotation);
}
if (nextWall > 0.0f)
{
    nextWall -= Time.deltaTime;
}

You can find the answer you want here
Edit according to @Commoble
Update updates once per graphical frame; FixedUpdate updates at a fixed interval set in your project settings.
Note: You can still find the answer in the link I gave you.