Use both. From position of cover, use Weaver as you minimize your exposure. When moving or when you lack cover, use Isoceles. Like Hemphill says, it maximizes use of your vest. After all, why expose the unprotected side?
At a recent instructor's conference, we learned of one instructor who trained for years and fired thousands of rounds using the Weaver. When startled, yep, just like Col. Applegate taught, he resorted to Isoceles. Bruce Siddle wrote an interesting article about human response under stress and why Isoceles works better. In short, we tend to face towards the threat and using Isoceles capitalizes on it.