If you are just creating a simple tool window then I suggest going the Add-in route.
Both Packages and Add-Ins are ways of extending Visual Studio. Generally speaking they have the same functionality. Packages are a bit more powerful and allow deeper integration into Visual Studio. But that deeper integration comes with a cost including bigger ramp up times and installation procedures.
Add-ins are designed to be a more light weight extension mechanism. Smaller ramp up time and easier installation.
If all you are doing is tool window with basic interaction of the editor our code model then add-in is the best route.