I don't see it as either/or. For decades, I've taught Isosceles AND classic Weaver, with Ray Chapman's modification of the Weaver stance (gun arm locked, support arm bent and pulling back, shoulders somewhat forward) in between.
The reason is, we have to be able to respond to unpredictable angles of attack, which are highly likely to come unexpectedly from a flank. Classic Weaver with both elbows bent gives you much more rotation range of the gun toward your non-dominant side than Isosceles, and Isosceles in turn gives you more range toward your dominant side than Weaver. You may be butt-down, caught in a narrow space between a couple of vehicles as in a parking lot, on ice or staircase or whatever, and be unable to step into your preferred stance.
What you're wearing may have an effect, too. Very heavy winter clothing or tight suits can restrict you from extending into Isosceles, but Weaver will work in that situation. Classic Weaver blades the armpit and side vulnerably toward your opponent if you're wearing body armor, but Isosceles squares up the armor between your torso and the identified threat you're facing, maimizing your protection. There are also issues of eye dominance, arm injury, ranges of movement, etc. to consider.
Space is limited here, but you'll find more discussion on this in a couple of my books, "StressFire" and "Gun Digest Book of Combat Handgunnery, Volume 6," both available from Amazon last I knew.