@davper said it short and sweet, WDW is Magical and UOS is Thrilling.
In my opinion, Universal I believe is more innovative in creating new attractions than Disney has lately (specifically in the creation of the new Harry Potter attractions specifically). A lot faster in doing it too.
I also want to share the little differences in the actual experiences themselves that would affect your experience:
- Very few character meet and greets compared to Disney, but are more unique at Universal. Some of the characters you'd expect to meet are surprising.
- Not really any "PhotoPass" type of individuals at Universal compared to Disney (do not know what they're called at Universal).
- You have fast pass at Disney for free, but have to pay for EP at Universal, or stay at an on property site in order to get unlimited EP.
- Disney's night time shows at all parks are absolutely amazing and worth seeing at least once. However, cannot judge Universal's show, as I haven't seen the new one yet.
- Theming for table service / unique dining is quite amazing at Disney (but that goes with the "magical" element as the ones that are extremely amazing are in relation to Princess theme), but while Universal doesn't have anything as extravagant, all their restaurants have great theming for their restaurants all around whether QS, or dine-in. Where as for Disney it's a hit, or miss in terms of them even trying with a QS location. It seems Universal does their best regardless of what type of restaurant it is.
- Quality of food at Quick Service type of restaurants (for the most part) are better than Disney. For dine-in options, I do not have an opinion on who is the best.
- A lot of lines at Universal are designed to stay indoors for the most park, or long enough that they stay indoors, where as a lot more of Disney queues can be outdoors, or are not long enough that they end up exceeding outside.
- Rope drop / EE is a lot more relaxed, organized, and not as aggressive at Universal compared to Disney.
- Also the first couple of hours are pretty low in wait times at Universal, but can be longer in WWoHP areas, especially if it's peak season. Where as Disney parks (depending on which one), can get pretty crowded quickly sometimes, especially during peak seasons.
- Disney is starting to seem always busy no matter what time of the year you go, only getting more intense during certain times of the year, where as Universal has a lot more down time seasons.
- Universal app is more detailed than Disney and goes as far as even detailing things such as dog rest areas for one example, as well as I feel is easier to use.
I think I should stop now, but I think I covered most it in terms of things that could affect how you feel, or experience the places themselves.