But it's not just about sentence structure, the plotting has improved as well. PS is a bit rickety- even though everything comes together in the end- but CoS is very well-made. It's like a good piece of furniture; the joints dovetail; you give it a good, hard smack and it doesn't wobble.
There's a certain lessening of joi de vivre. This happens with sequels. A first book is all "look at me, I'm dancing", a second book is about improving one's craft. This registers as a darkening of tone. There's less invention. Take the denouements: a guy with a face on the back of his head is more original and more fun than a big, old snake. But, never mind- CoS is still compulsively readable.
And now for The Prisoner of Azkaban.