Hi! We've been taking our kids to Universal since the eldest was 3 years old and the middle child was 2 months old. They were 12, 9, and 6 on our most recent trip. And 10, 7, and 4 on the trip to Universal before that! We've gone to Legoland for one day every year for the past 3 years as well, including this year - and starting with the 10, 7,and 4 trip.
Universal is absolutely amazing. Legoland is a lot of fun too - but I don't think it's worth close to the regular ticket price. (We use the homeschool discount with makes it worth it). On our first two trips to Legoland, we were there from open to close and could have easily spent more time there. However, this last time, we were ready to leave an hour before close. My kids still enjoyed it - even the 12 year old - but it's not on the same scale as Universal.
My advice is that if your eldest is into Legos at all, do Legoland. I do think Universal is more amazing but Legoland is a good park and you may be limited in the number of years that it will work for your family. If you think your eldest won't be into Legoland at all, then do Universal. We did 6 days in the Universal parks when they were 10, 7, and 4. (And again, returning today, and they are now 12, 9, and 6). My youngest has sensory issues which limit rides in many places. He refused to the rides at Legoland at 4, but did MUCH better at 5. At 6, it worked great. As for Universal, he refused rides at Universal at 4. (And refused most of Disney rides at 5 until our trip was almost over). At 6, he did better but many were still too scary or challenging for his sensory sensitivities. So - it meant doing things like letting him ride Woody Woodpecker 10 times in a row to make up for his waiting. He really liked the ball place by Curious George too and the Flight of the Hippogriff. There are several play areas too he liked - like the Jurrassic park play area and the play area next to Woody Woodpecker. (I think it might be Fieval's playland). He agreed to do MIB, but only with his eyes closed so he wouldn't see the aliens. He did this 4 times during our 6 day trip. If my youngest hadn't been a sensory-quirked child, he would have done a lot more. (My eldest was doing Spiderman at 3. My middle child did get freaked out on Simpsons at 3 but did well when she did it again a couple years later. My 6 year old isn't ready to try Spiderman and once was enough for him for Simpsons).
