We stayed here for a week and loved it and having also drunk in the mayfair bar i can do a fair comparison - my Hotel Langham review is also on my blog (link below) but here is my summary:

The mayfair - bar super trendy media types, LOUD music and i like music but this was loud, felt abit like a showey offey meat market to be honest (personal opinion here so dont bite my head off) though undoubtedly very trendy, was not my cup of tea. Look was modern black marble floors (from memory) floor to ceiling windows. Oh dark candles and good free nuts!

The Langham bar- really good though I only briefly went in there as they wont allow children and we travel with my boy. Its in a beautiful old victorian building with a cosy atmosphere, dark wood floors, white/cream walls, atmospheric bar, candles. lovely view overlooking the BBC. We saw fiona bruice and Jeremy Clarkson in there. I thought this bar is really superb. and Raffles is right, the Roux restaurant is also superb.

I personally would always choose the langham over the mayfair although i would argue that for designer shopping the langham is slightly less convenient. The mayfair gets some iffy reviews on tripadvisor. We booked a club room at the langham and the food in their club lounge is outrageously good tho club isnt cheap- we felt worth it. Spa and pool were excellent and the leaders club benefit when we were there was about £80 to spend in the spa which was nice.

Rooms- we got an upgrade to a suite which was fabulous. but as the building is old and traditional the base rooms can be quite small and traditional in style; anyways see my review in more detail here and if you have any questions do please PM me
http://www.luxurytraveldiary.com/201...angham-london/

We also just stayed at the Four Seasons London which we loved.. also reccomended