I've owned a Browning BDA and a Sig 226 (W German made). I currently own all newer Sigs (220R Stainless Elite, 220R, 220R Carry and 229R).
My pistols with the folded slides did feel a bit better in the hand. They don't feel as top heavy. However, I really believe that is the only area where they are superior to the guns with milled slides. There is no difference in reliability that I have seen. The milled slide guns are easier to keep clean and have reduced susceptibility to corrosion. I also think the frames on the older guns had some finish issues. My 226 developed an inexplicable browning in its corners and nooks-n-crannies (the color of rust) and lost finish on the inside of the dust cover. The inside of the frame had an iridescent hue that looked like oil on water. It was never problematic, but it did look odd. And I've seen other older Sigs that looked that way.
In terms of quality, the older guns were already well developed when they were imported into the US. Newer models had hiccups until their anomalies were worked out. I think this produced a perception that newer guns were less reliable than older ones. But I doubt that's the case. I haven't seen it. I actually prefer the new P series guns to the old ones.