The brand is always a factor. US and UK are typically more brand loyal to Xbox and you can see that sales are somewhat more even there.
Can you explain why 360 did do well in US and UK then?
After all Xbox brand was weak and Playstation brand huge thanks to PS1 and PS2.
If it was about brand 360 wouldn't have stood a chance.
Clearly other factors such as price, perception of which plays games better, stability of online infrastructure play a part.
Mind you PS4 is on course to be clear winner whic would give Sony three big winners out of four generations and a decent effort with PS3.
Maybe it is all about brand but the mistake is thinking the Xbox brand is the stronger?
Myself I don't think US or UK brand loyalty is that strong with mass market. I believe price and market perception are the two key pillars and PS4 was the clear leader with those.