I go solo often, 65 year old. Hubby hates to travel, son has his own family now. I went 3 times last year, two of those were solo. I've been once solo this year and will going back in August solo. My son and I have been going since he was just shy of 3, he is 41 now. Most years until he was adult, just he and I. Some years my Mama went with us. After he became an adult a couple of times his girlfriend/wife (different women, wife now ex) came with us. About 10 years ago I started going solo because it was safe and at the time I was looking, actually less expensive then the other places I would go solo.
I have not been to Universal solo, or even within the last 10 or 15 years (or more) so I can't speak to that.
I love my WDW solo trips, dare I say maybe even more than my trips with my son. I tend to do split stays, a couple of days at a value then move over to an Epcot resort. I'm guessing the last 7 trips have been at the Swan/Dolphin just because it is way less than the WDW resorts but I've stayed at all the Epcot ones. The August trip will be a split with 2 nights at the Swan and 4 nights at the YC. I don't plan on going again until probably next April and I'm probably going to end up having to stay at a moderate or Swan/Dolphin if I can get that for less, so I'm going big this trip and staying at my all time favorite resort, YC.