I believe the check valve is best located at the diffuser.

A check valve will be more likely to stop the backflow of water (at the diffuser) than the back flow of air (at the pump). Air will leak through a small gap more easily than water will through the same gap.

If you had a quality air check valve at the pump, it would serve the same purpose so long as it's "air-tight" at low pressure (about 3-5 psi).

The purpose of the check valve is to keep water out of the line which eliminates debris from entering the system and avoids any freezing and bursting issues.

Fish on!,
Noel