Business trip in December - 1 or 2 days on my own to see the Christmas magic - London or Amsterdam?

I don't have any Amsterdam experience but I love the lights in Carnaby Street and along Regent and Oxford Streets. The bonus of this area is looking at all the window displays in the shops - and definitely take a stroll through Liberty. I really enjoyed the Leicester Square Christmas Market and the one at Southbank Centre was ok if not exciting. I wasn't a huge fan of Winter Wonderland in Hyde Park though many will direct you there.

this is a dup of another where my answer is

No need to have a taxi tour - the lights are basically confined to Regent Street and Oxford Street. Carnaby Street which was mentioned is close to Regent Street and is pedestrianised.

In London, I’d simply find one of the regular service double-decker buses that happens to go along Oxford Street and Regent Street and sit up top rather than doing a special tour. Then getting off the bus, walking the route in reverse and looking in the windows.

It’ll be dark by 4pm so plenty of time to see the lights.

One can see the lights walking between Piccadilly Circus and Oxford Circus then to Bond St. From Piccadilly, walk up Regent street. There are signs for Carnaby street (Liberty London). After that minor detour to see those lights, go back to Regent St to Oxford Circus. Then walk up Oxford St. I’m in London now and the lights are already up, although the ones on Regent St. were not turned on yet. Those lights are angels. I had hoped they would have beeen turned on. Oxford street is about the same as they were in 2015. Carnaby has a Caribbean theme with palm trees, parrots, etc. light. Very nice. Walking the entire route at a leisurely pace takes about an hour.

Regent Street lights are turned on this coming Thursday evening, with a free pop performances.

Much more sedate and subtle are the decorations in Burlington Arcade which are always worth a visit - just off Piccadilly close to the Royal Academy.

Thanks everyone for the great suggestions. I've decided to spend my free time in London and are looking forward to seeing the Christmas lights and market!

"definitely take a stroll through Liberty."

MNFerst beat me to it - when I was there in Christmas 1985, Liberty was beautifully decorated, and apparently it's famous for this. It's also smaller than the other department stores like Harrods.