Every tick does not scale as the framerate gets lower. So, if your framerate is 10fps, an every tick action will seem to take place at 1/6th the proper speed.
On the other hand, dt scales to lower framerates. Thus, actions will appear to happen at the same speed, though dt controlled animations may look very choppy.
Generally, dt is preferable. However, for some games (for example, puzzle games) it is preferable to handle things with per tick logic, as the timescaling effects of dt can cause glitchs such as collisions not registering.