I'm far from being an expert on the difference between UK and US trained PPLs. But having just got back from the US I do have some opinions on this.
I was impressed with the actual standard of flying over there. People with the same number of hours as I have knew lots more about mountain flying, and had practised autos etc from 500 ft, which I'd never done. I did a lot of that kind of stuff there with an instructor, and learned a lot. But then I realised I have little use for it here. We don't have any mountains higher than 4000 ft or so, and that only in Scotland. We don't have any high altitude airfields. (Yes, I'd forgotten how to use the density altitude chart; hadn't used it since my PPL skills test). We don't fly around at under 500 ft because it's illegal; it's also unsafe because you're liable to encounter a highspeed military jets.
On the other hand, PPLs over there didn't seem to have a clue about navigation. People seemed to follow roads or coastlines all the time. I took a student who was just about to do his checkride with me when I flew from LGB to Santa Barbara. We got lost in LA, trying to follow the freeways. He looked around in desperation, then got out his GPS. I looked at the compass and which direction we were heading, noted where the coastline was, worked out my last known point - basic navigational skills; it works even in LA. But he didn't seem to even know how to try that.
Then there was weather. If the wind got up to 12 kts over there, people got worried. I did my first solo over here with 17 kts. And I think a lot of US PPLs would have problems with the kind of weather we take for granted in the UK.
I doubt if I could pass a US commercial checkride without a bit of extra training; I don't have the experience of mountain flying and suchlike. But I don't need it here. But I doubt if some of the US lowish hours CPLs would pass the JAR commercial skills test, certainly not the nav, finding an isolated farmhouse among many similar ones, having been given a gride reference in advance. But they don't need to. They don't do that sort of flying over there.
To summarise, it seemed to me, after admittedly very limited experience in only one area of the US, that you learn what you need to for where you're going to fly. At least at first. Neither is better or worse, but they are a bit different. For that reason, I think that initially learning in the country where you intend to fly is a good idea. After that going to other places to get different experience is good (and great fun, if you don't mind feeling like an idiot for a bit!).