They are both good reflectors. The Lumen Max is designed to throw a wider cone of light and is better for tank foot prints greater than 24". If you have a wide bottomed tank, the Lumen Max might be a better choice. If your 75gal is 24" or less in width, the LumenBright would be a better choice.
Mike at reefspecialty.com would be a good person to ask which reflector would be best for your application. He can give you good guidance on how high off the water either reflector should be mounted. The LumenBright needs to be 16-18" off the water to focus the light appropriately.
Mike M