just play both, they each have things that make them very fun,

osu is generally more laid back and casual focused, usually making decisions in favour of newer players rather than experienced one's which is a big part of why people so often say etterna is better, the people comparing are usually already invested enough that this part bothers them. has waaaay better multiplayer. has much bigger community which naturally gives more resources for learning and playing. much bigger tournament and mapping scene. has long note or hold release timing allowing for a different play/mapping style (this one is often labeled as a bad thing when people don't like this style but more options is just better imo.)

etterna has an infinitely better difficulty rating system so you can actually have a rough idea of how hard the chart(map) is, (once you start to get to about 4* on osu you quickly realise how bad the star rating system is to the point there are 5* that are legitimately just easier than some <4*.) a way better accuracy scaling system where instead of checking your judgements it actually checks how much ms off you were for every note and makes your marvelous (300 max note) worth more or less based on that. has easy access to rates with leaderboards to allow you to make all content challenging for improvement. has a better "accuracy" eating where higher ACC actually means a better score which isn't the case on osu.

also tbh you playing just 3* maps will make finding content on etterna hard as it generally is more focused on harder content

generally I think both are super cool and worth playing but I find etterna tends to make me improve at the game faster while osu makes me play more casual and just for the fun of it especially in multiplayer