One common misconception is that the Harry Potter lands and rides are at Disney. They are not, they are at Universal Studios, which is a separate resort that requires separate tickets.
I haven't been to Universal near as much as Disney, but I do have a few tips and tricks for those considering paying it a visit:
There are no discounted combo tickets to Universal and Disney, and if you see someone purporting to sell them, you should be suspicious. Travel agents can of course book you tickets at both locations as part of a package, but you'll be paying full price for both. Both resorts also incentivize you to do extended stays by making tickets less expensive the longer you stay. The corollary to that is that the per-ticket cost is the highest for shorter stays, so if you're doing a trip where you're doing a day or two at Disney and a day or two at Universal, you're going to be spending more than if you did 5 or 6 days at a single resort.
Universal's version of Lightning Lane is called Express Pass. It allows you to wait in a shorter line at a significant number of attractions, including most of the headliner attractions. You can either pay for it, or a version of it that allows you to use it once per attraction, per day, is included for free at the Portofino, Hard Rock, and Royal Pacific hotels. The cost varies depending upon how crowded it is, and can in some instances be more expensive than the park tickets. In fact, it can sometimes be cheaper to just get a room at Royal Pacific even if you don't intend to use it to take advantage of the free Express Pass. For a one night hotel stay, you'll have free Express Pass for the day you check in and the day you depart, and for a family of four, a $400 hotel room for a night could get you two days of Express Pass benefits worth $1000 or more.
Generally speaking, Universal is very thrill-ride forward, and most of the best attractions are things that will spin, shoot, twist, turn and drop you. I personally find them amazing, but if you're the sort of person that doesn't care for motion simulators or roller coasters, take a look at the attraction listing before you go and make sure there's enough there to warrant the trip. Part of the reason we don't go as often as we go to Disney is that there's not nearly as much to hold the interest of my family that doesn't do thrill rides.
If you are going to Universal primarily to experience the Harry Potter areas and attractions, be aware that you have to have the Park-to-Park ticket to be able to ride the Hogwart's Express. It travels between the two parks, and you will not be able to board unless you are able to enter the other park.