I haven't been to Legoland (although it has been my to-go list, but everytime we go, we don't have a car and Legoland is bit far away) but when we went to SW last year, my DD (she was 4) loved it!
She usually really likes rides - and quite enjoyed them at WDW, but at SW, she didn't care that she couldn't go on the rides and kept on watching the shows after the shows! She loved all of them and we also made sure we talk about how important it is for us to cohabitate with the animals, recycle, etc.
It might be more so, because we went in December and there are more shows during Holidays, I was told - like ice skating shows, Sesame Street Christmas shows, etc.
We're going back tomorrow and can let you know how it goes with new Sesame Street Land and other new rides.
I also saw a small splash pads area for kids and a parade.
As far as Legoland goes, I was told that it is great for younger children, so we plan to go next year. Given my DD loves roller coasters, but she is not quite at that height yet, we want to wait until she actually reaches the height requirement for that and go, so we do not disappoint her. Both my DH and my DD LOVES
Lego (like DH wants Lego for his birthday.. hm...), but at this point, I think she likes animals more. According to my friend who went, told me that they spent more time at the Legoland hotel that night building Legos than at the Park itself. But her son (who was also 4 when he went) does not like rides, so that explains.
If you go in the summer, you also can go to the waterpark, which is a plus, I think. We always travel in winter, so their waterpark is always closed.
Lastly, my DH and I watched Youtube videos of both SW and Legoland parks by several people and discussed what would work better for our DD before we made our decision.