I prefer Charizard X though both are excellent designs. I think partially its down to gen 1's lack of Dragons and anything that adds more dragons to regions that are severely limited in Dragons works for me. Charizard X meant Lance could actually have a Kanto based team with no repeats and half the team are Dragon typed (his rematch team in let's go) and for me I quite enjoyed those battles. Though I will admit Charizard-Y's ability to set up sun is useful.
Y, as said before it feels more like a natural evolution to Charizard by emphasizing its flying nature via its sleek and aerodynamic design. X to me doesn't really change too much of Charizard outside of giving it a different color scheme and some spikes.
I like Y more but both are great. The reason I prefer Y is because it feels more natural for Charizard's mega and X just looks to different, more like a regional variant than a mega form. I think they just felt pressured to make Charizard into a Dragon type like fans always wanted so they went full "fan service" mode and even made it black like the beloved shiny form so it doesn't feel right as a mega for me. But hey, at least I can't say that it doesn't look super cool because it does.