Stating some obvious stuff that may be less obvious if you haven't been there in 25 years.

Islands of Adventure is a separate park that opened in 1999 and feels more immersive (like magic kingdom) than the original Universal Studios park (think MGM/Hollywood studios). The previously mentioned harry potter train (ride) can take you from one to the other, or you can walk - takes ~10 min from entrance gate to entrance gate (but can be quite a bit longer if you're going from the back of one park to the back of the other). As such, you probably want to do a whole day in one park or 1/2 day in one and 1/2 in the other. These parks are MUCH MUCH closer to each other than any Disney park is to another.

Even after 20 years, Spiderman (Islands of Adventure) is probably still my favorite. Transformers (Universal Studios) is like Spiderman 2.0 (but not as good, IMO).

Staying on-site is the key bit. You get an express pass which gets you access to a much shorter line for each ride (but there definitely can still be a line! can be 30 minutes+ vs hours without the express pas).

Enjoy! It is a really great experience.