Which high street brands do you want? The extra travel time to Westfields might be offset by the walking time around Oxford Street if, say, you want to go to M&S (Marble Arch) and Waterstones (Piccadilly).
It's always late night shopping every day and all year round on Oxford Street and Westfields! John Lewis tends to close earlier (8pm I think) but most shops are open until 9pm, some 10pm, especially close to Christmas.
If you want high street brands, is it worth the hassle? Prices will be the same online from the same stores, and for brands that aren't store specific you will probably find cheaper prices for the same things online if you search eg toys, homewares, toiletries, perfume. Personally, I would be heading to somewhere like Marylebone High Street, Kings Road, Covent Garden, the back streets off of Oxford Street, Soho, Camden Market, Spitalfields, Brick Lane, a Christmas market etc where you might find something a bit different.
If you just fancy going because you think it would be more fun than online shopping, I would pick the most convenient area with the best atmosphere.