I think both ways are alright, depending on prefernce. Every horse trainer has a different view on training.
To me, riding with your outside leg IN FRONT of the girth is wrong, ineffective, and confusing to your horse when trying to teach leads.
I ride at a canter/lope with my INSIDE leg on or slightly in front of the girth to have actual contact with the skin of the horse.
This helps encourage the horse to arc around your inside leg as you are making the circle, therefore creating a better shape to the circle, and bending and lifting the rib cage.
I ride at a canter/lope with my OUTSIDE leg BEHIND the girth.
You should always cue for the lope with your outside leg if you are going on a circle, to get your horse to pick up the correct lead (which would be leading into the canter with the horse's INSIDE leg.)
If you cue for the canter with your outside leg and lift up on the inside rein, it helps the horse's body be more easily positioned to take the inside lead.
This also helps encourage the hips to remain in the circle, instead of swinging out, which will ruin your horses form, most likely make the circle mis-shapen, and more often than not, allows the horse to more easily drop his shoulder to the inside. When he does this, he loses the lift in his front end, which causes him to lose his collection and the impulsion from his hindquarter. Overall he falls apart as far as being collected and using his body correctly goes.