I used to use Pygame when I started out making games. I think it's a great tool. Unity is a different kind of tool though, it's a commercially support integrated development environment. I think that there are different cases for using both. If you want to get something together quick with little overhead then Pygame is a great choice. If you want to make a 3d game then Unity is going to come packaged with a lot of the code and functionality you'll want. Personally I don't use Unity (there are plentry of people here that don't), I use love for making 2d games.