I've taught for a long time, shot in serious competition for years, have attended many firearms training programs and have been trained in many shooting stances (starting from the old FBI crouch through the modern Isosceles). What I've learned over time is that as long as the shooting stance used is a stable, comfortable position that allows some flexibility in movement and offers reasonable firearm control, it really doesn't matter which one you choose for yourself. Weaver, Isosceles, or modifications thereof, all cover the necessary aspects of a good shooting stance for defensive use.
I found it amusing, while assisting at a Reserve Officer Academy, that the new defensive tactics and firearms instructor insisted that the cadets use the bladed interview position when making contact, thus keeping the holstered firearm further away from the person contacted (and I find this reasonable), but then insisted that they shoot from the Isosceles. I pointed out to him that since the officer would already be in a bladed position wouldn't it make sense to draw and fire from that same bladed position (i.e. Weaver Stance) since this would minimize movement and the need to learn/train two stances? He then tried to lecture me on why the Isosceles was the "proven" better firing stance (it has yet to be "proven" to my satisfaction). Fortunately for me the Chief Instructor interjected saying that maybe the new instructor should note that I was a more experienced and better trained instructor/officer than he was. Oh, and I out shot the new instructor (he using a Glock 17 and his isosceles position, me using a Ruger GP 100 and my modified Weaver) in our recent re-qualification.
Both the Weaver and the Isosceles (and most of the modified versions) are equally effective. Both have some minor advantages and minor limitations. The one that works best for the shooter (the person it should matter to most - not the instructor) is the stance they should concentrate on learning well. Try both. Pick for yourself. I use a modified Weaver - it works for me. I teach several and have the individual student select the best one for their self.