LANGHAM place is more 'local' as there are a lot more residents living in Mongkok than tourists there and it is VERY crowded. Densest place on earth I think.
Closer to Temple Street, Ladie's Market, Nathan Road.
Four Seasons is in central, more business area, not so busy more 'western' as there are a lot of expats living in Central.
Closer to the peak, star ferry, lan kwai fong.
I personally think the 4S is better if the price is comparable, as the rooms in Langham place didn't impress me that much. Plus the 4S is in what I consider a more comfortable area without all the hussle and bussle of HK right outside.
But it really depends if you're looking for a more HK local experience where you are immersed in it all with all the food stalls, shopping, and smells, or want something more relaxing and a bit removed and not as convenient to all that as the 4S you need to walk a bit further to most things.