Both have wonderful qualities but for me it is Universal. I don't have any younger kids anymore and my style of vacation is much more laid back. Also I am a huge Harry Potter fan!
Our last trip to Universal was wonderful. We stayed at Royal Pacific and had the express passes. Wow was that a game changer! Loved not having to queue up and it was nice to get into the shows and have better seats. I also loved that we could walk to everything and not have to wait for a shuttle to get us around. We could do one park in the morning, go to Citywalk for lunch and then head over to the other park. If one park seemed crowded we could just walk over to the other one.
We were beyond delighted with the express pass.
Concerning City Walk and the other dining options, Universal has really good restaurants AND they make it easy to spend money at them! Even when the restaurants were full, we rarely had to wait more than 20 minutes for a table. It was always "yes, give us a few minutes" unlike Disney were we walked up to half-full restaurants that were turning away people because they didn't have an ADR.
I prefer Universal for many of the same reasons other posters have mentioned. Staying on-site with EP is so much easier than trying to plan out 3 FPs each day. Not to mention being able to walk to the parks for half of the price for the privilege of doing that at Disney (Does that make sense? I have read it three times and I'm not sure how to reword it! Staying at Cont. cost twice as much as RPR, yet I can walk to TWO parks from the RPR)! Being able to make last -minute ADRs is a blessing too. Because we refuse to go to Florida in the summer or over holiday breaks, our trips are short. At Disney, I feel like I have to pick and choose what I want to do and then miss out on so much. I don't feel this way at US. I can do two parks in two days and feel like I've done all I wanted. Overall, US is just a more relaxed vacation.
If you asked this question over on the TPAS board, you would get a whole lot more people who prefer Disney. I bet I could guess their reasons too.
It makes sense to me! I grossly underestimated the amount of transit/commuting time a Disney vacation requires if you want to park hop or just go to the parks if your resort is far away. My husband NEVER asks about price or complains about cost when we do a family vacation as long as he is relaxed and having a good time. Cost came up a few times at Disney in relation to cost vs. enjoyment.
I don't mean to dump on Disney and people's love of Disney, they are two very different experiences both in the style of park and food and lodging options. We just happen to prefer Universal more.