I've used both in my charger. I liked them,but I'd give the edge to M1 only because at cold start the engine felt like it had more pep,almost like there was less drag,but both are great products.
There was a used oil analysis posted within the past few days showing in his vehicle the Castrol had better tbn retention vs the m1 and was able to run more miles with it,if you are trying to maximize mileage on your oil.
I hard cap my vehicles at 10000 miles,with a quality synthetic.
If you've got no particular preference buy which is cheaper.