Here are some brief differences I wanted to point out from golden age Captain Marvel and the Captains Marvel of The Power of Shazam! In the golden age Fawcett comics, Billy Batson and his sister Mary are known as Captain Marvel and Mary Marvel respectively. But in The Power of Shazam! series, Billy and Mary are both called Captain Marvel! In The Power of Shazam! #27, the wizard Shazam reveals that both Billy and Mary are to have his powers as the Captains Marvel.
Another difference you'll see in The Power of Shazam! is that when Mary transforms, she transforms from a teenager to an adult (and rumored to look like her mother, Lyn). In the golden age when Mary transformed into Mary Marvel, she remained as a young girl.
These are three differences from the golden age Captain Marvel, but I enjoy the differences and Jerry Ordway does a great job using them to enhance the stories and add more depth to the characters!