LOL Personal preference...
Android box has a lot more flexibility, but is a lot more complex to setup and keep running. Plus might be an additional remote.
SmartTV, everything is done on the TV but the add-on applications are very limited and are based on brand and model of the TV.
Depends on what (and where from) you want to stream on it. My own entertainment setup has wired Ethernet (everything that can be wired is here), but yours may not. Check that your selected SmartTV has WiFi if needed for you, many of the Android boxes do have WiFi.
It also depends upon what you want to do to need a smart TV. It might be just as easy to connect a Chromecast if you are just trying to stream content.
Much like kevinds mentioned, certain SmartTVs have their own custom functionality and limitations. If you were hoping to match with your Android phone or tablet, you probably want to look for a TV that says Android TV which limits your options. That's not to say alternatives like Samsung are not good, its just different.