Either or both depending on your habits and physiology.

Glutamic acid (glutamate) is not an essential amino acid in the sense that you need it in your diet. Though it has many vitally important roles in the body, you can make us much as you need if you get the proper complement of essential amino acids etc. Still, you can use it in the diet.

The catch is that among its functions, it is an excitatory (or stimulating) neurotransmitter; it passes on nervous messages, and if you have too much of it in your blood it can cause nasty symptoms in a healthy adult, and cause nerve damage if you really overdo it. That generally does no more damage than a bit of one of the forms of "Chinese Restaurant Syndrome" (another is Bacillus cereus food poisoning, which is altogether different). Some folks have been arguing that MSG was innocent and the syndrome was caused by NaCl, but I am a bit sceptical about that. 

But what is far more serious is that MSG can cause long-lasting damage to growing nerves in infants. Personally  I am cynical about the panic about forbidding the use of MSG in foods for adults, but I am all in favour of forbidding it in food for infants. It only was of value for pleasing the taste for the mothers anyway; the babies didn't seem to care one way or the other.