Different things for different purposes but it is almost always best to assess with one method then confirm or not with the other method.
In many cases, neither can be trusted but we know the eyes can lie to us.
For example, your eyes can trick you into thinking you see three dimensions, when in fact the metrics show it's really just a 2 dimensional drawing.
The eyes can be fooled and it happens to people every day. Some say numbers don't lie, but they can be misrepresented and show the wrong thing if not used correctly.
It really is best to confirm with metrics, even eyewitnesses are some of the most unreliable pieces of court available evidence.