The two main points have been stated. A lower layout, with in reason say 30-40 inches, is easier to work on. A higher layout, say 48-60 inches is more fun to look at the trains. So there are a lot personal choices and thoughts that go into picking the height. I find working a yard and having to read car numbers back 24 inchees from the aisle is much easier on a layout 35 inches or so high. But I really like seeing a train go by pretty much at eye level.
An advantage of high layout if it is around the walls in a room, is that entry and exit may not require much ducking. And wiring u nderneath is easier to get to on a tall layout. But a derailment more than a few inches from the front on a high layout will probably require a stool to remedy.
The best advice is to pick a height you like and comfortable with.
I would offer one more thought, if you are building a layout that will last for some years, don't make it low for the grandchildren. First they grow tall and fast. My grandson who was a toddler when I was working on the layout is now taller than me. Luckily I did not size it for access by a 3-4 year old.
