Of the two OWB holster types you mentioned, the pancake and the Askins Avenger, I would agree with other posters that the pancake holds the gun closer to the body. Having two belt loops, one on either side of the gun, seems to pull things tighter.
But I would say that the tightest fitting OWB holster is one in which the belt goes over the gun and holster. The force of the belt, as it is tightened, pulls the gun into the side very firmly, much like the steel hoops on a barrel hold the wooden slats tight against each other. Several makers produce a holster like this, such as the Mitch Rosen "Thinman" shown below:
