Very interesting question.
IL now tends to look less and less like the one in CA because they're owned by different people. Waay back when they were both owned by Marriott, they were quite similar.
On the plus side the entrance area with the long reflecting pool and the beautiful double decker carousel remains the same. That should help bring back some memories. If you stay in that area, you're set!
Unfortunately, after that point, they begin to diverge and things are quite different.
The park in Gurnee IL belongs to Six Flags now and has thrill ride after thrill ride. Though the Whizzer's still there.
The park in California is now part of Cedar Fair (who owns Cedar Point) and is a little more show oriented. Their rides, while very good are in general a little more tamer than the ones in IL.
For example, on inverted coasters, IL has Batman which is a very fast and thrilling ride. In CA, they have Top Gun (now Flightdeck) which is also inverted, but paced a bit slower with a bit more time between elements.
Anyways, hope I helped answer your question.