Thank you all for explanations and suggestions.
I will go with Parallels, as it seems to me that with Bootcamp it would be both annoying and a big waste of time to have to restart the computer every time I want to play.
Then I am not a proper gamer, GW2 is in fact the only one game I play, and I prefer a quick access and switch to Mac environment when I need it.
Windows may also be useful for other use in the future.
So basically I'm going to try the Parallels configuration, that is more simple between the two, to understand if the performance degradation will be acceptable to my way of playing, that is not a top one, and if not I will seriously consider the recommendations about a configuration with Bootcamp.