.aab is the way to go if you keep in mind that our support for .aab files is fairly new. I think there were a few early reports about issues with .aab files and some people are sticking with .apk files for now, but for others they are working just fine.
We originally discouraged people from using Google App signing because frankly we knew little about it and giving up control over that process was hard, but in testing .aab files, I’ve gone through the process and it’s not that painful. They have a really good walk-through to get your keystore processed correctly.
.aab files will get rid of the “This app isn’t optimized” warning. They produce overall smaller files than a .apk with both 64-bit and 32-bit support in them.
Rob