I can help you on this, as I live right next to Disney and Universal and go multiple times a year! Obviously, I suggest both, if possible. The parks are so completely different from each other that comparing them is just not possible. Here is what to expect at both parks, as well as the environment.
If you’re okay with being in a VERY large crowd, and very long line waits, and more of a ‘child magic’ feel, then Disney is for you. Disney is huge-it has four parks. However, you will have to pay for each individual park, at about $100 ticket per park. So if you want to attend all four parks, that’s $400 for one of you alone, not including food, drinks, etc. However, Disney IS magical, and the rides are very nostolgic. It does feel magical there (I’ve been going for years) and if you’re into the more tame, relaxed rides, then Disney is your choice.
Universal is the much more cheap option, because there are two parks (and I highly suggest going to both, especially if you want to go to Harry Potter world. Diagon ally is in one park, and Hogsmade in another, and if you want to ride the train, you need to buy passes for both parks). But, still, much more cheaper then Disney. If you want a more adult friendly environment, with more intense, complex, and a wider selection of rides, Universal is where you should go.
Disney is great-but Disney, honestly, can just come across as very forced, money hungry, and very dissapointing at times. I have NEVER been dissapointed at Universal, and if Harry Potter is your thing, you will be in LOVE. So I suggest maybe choosing one or two parks for Disney (if one park, I obviously say Magic Kingdom-cut out epcot in the options because unless you’re drinking around the world-which is SO expensive-there’s nothing in epcot) and then spend the rest of the time at Universal. For the record, Universal can be done in two days, tops.
Hope I can help some! 🙂