Generalizing a bit, but the east coast in the summer is often very hot, humid, and buggy. The east coast fall is often less humid and more temperate (more pleasant to be outside).

The west coast summer can be hot, but is rarely humid.

Middle of the country varies too much to really generalize imo.

Depends on a lot of things of course--including where on the East Coast and the activity. (I wouldn't make such a categorical statement about Maine for example.) But not so hot and humid. Not so buggy. (Also fewer people.) In general, I'd say that September/October are the ideal months for much of New England (and a lot of the West Coast as well.)