Selection is not longer an option as Thunkable has firstly removed access for new users to the Classic variant, and secondly announced that support will be terminated by next summer.
The URL for Classic is “app.thunkable.com”.
Your options are thus limited to go with X or to take your business elsewhere – like Kodular – if you insist on an environment that is similar to App Inventor.
The bottom line is that App Inventor, Kodular and Thunkable Classic can only target Android, while X can produce app that will be available on iOS as well.
The flip side is that X is missing several key components that many Classic developers depend on; that is why many of us will NOT go with X, but are moving to Kodular instead. And perhaps one day, Kodular would support iOS as well.
For the record, an AIA file from App Inventor or from Classic Thunkable can be loaded in Kodular and would require minimal adjustments to work (since some components change in the number of arguments).
It is your call.