We should use Gtk.Grid because the reference doc say that Gtk.Box could be deprecated in the future :
"gtk_hbox_new has been deprecated since version 3.2 and should not be used in newly-written code. You can use gtk_box_new() with GTK_ORIENTATION_HORIZONTAL instead, which is a quick and easy change. But the recommendation is to switch to GtkGrid, since GtkBox is going to go away eventually. See Migrating from other containers to GtkGrid."
We should use Gtk.Grid because the reference doc say that Gtk.Box could be deprecated in the future :
"gtk_hbox_new has been deprecated since version 3.2 and should not be used in newly-written code. You can use gtk_box_new() with GTK_ORIENTATION _HORIZONTAL instead, which is a quick and easy change. But the recommendation is to switch to GtkGrid, since GtkBox is going to go away eventually. See Migrating from other containers to GtkGrid."