Apparently the discolouration ended up being from some cases that people were using, with the dye temporarily 'sticking' to the iPhone (which would obviously be more pronounced on the white one), rather then heat. Plus the white model doesn't feature any hardware that's different to the black one, so it wouldn't overheat just because it happens to have a white backplate
Personally I prefer the black, it looks a lot more streamlined with the black on the front of the device, and it's contrast with the silver bezel, volume buttons and the logo on the back looks wonderful. To me, the white looks a bit out of place (I don't like too many colours, having just two looks great), but a massive pro is that scratches aren't really noticeable. On my black one, the few scratches I got really stand out at times.