I don't think that there will ever be a winner in the UK vs USA debate. Some will always claim that learning in the USA means not learning the British way or in British weather conditions, or that the training isn't as good because of the instructors, or because there's pressure to get people passed, or whatever. Others [like me] will always regard the British system as flawed, because people don't fly for three weeks or more (due weather maybe) and end up taking three steps forward at each lesson and two back inbetween, or worse are forced to repeat certain exercises (stalls, etc) if they aren't done for a certain period of time meaning you go over the same ground over and over again. Likewise, there are too many stakeholders with vested interests to draw a really objective conclusion - British schools and instructors regarding the USA as doing them out of business, US schools claiming they offer a good service for those on a budget or with a limited amount of free time, people who might not otherwise even get into GA.
Its impossible to really assess the results of the USA versus UK, because evernone learns at their own pace and its not possible to train the same person through each system from scratch. Equally, post training there's no mandatory checks in the system for two years (by which time some will have dropped out, and those still in GA will have changed to some degree), which means any overall generalisation about the raw output from flying schools either side of the pond cannot be reliable. I am certain, as suggested above, that each system generates its fair share of good and not so good pilots at the end of the process.
Training in the US showing no signs of going away, no matter how much some people don't like the idea. Whether training in the USA or the UK turns out good or not so good pilots, surely the objective post qualification is not to bitch about the system that produced them, but to keep them in GA, keep them flying, and keep them safe.
Andy