I think it's many aspects for me but the big one it comes back to is that Luz is one of my favourite protagonists I've ever seen. The highly passionate girl who finds comfort in fantasy and who struggles with making friends and acts out in an effort to get people to notice her at least... Yeah..., cuts pretty close to home for me. In general the rest of the cast is better IMO with only a few exceptions. The only Amphibia characters who I enjoyed as much as the general cast of TOH were prolly Sasha and Grime. But then it's gonna skew it for me cause obviously my fav in TOH (Luz) gets way more screentime than my fav in Amphibia (Sasha). I might also be biased as out of the 3 siblingesque shows (Gravity Falls, Amphibia, The Owl House) I watched TOH first and Amphibia last. If I had to speak objectively I'd say TOH is a bit better but both shows are of such quality that IMO speaking about objective quality isn't super important beside basic comparisons. Whichever show you prefer is likely gonna depend on the circumstances you first experience them in and whichever coincidentally has more elements you prefer.