Based on comments here on Beoworld it's safe to say that the S8 is identical to the BL14.
The only difference is the colour, and the difference in accessorize (type of stands) and the option to buy more speakers .
It should be noted though that the BL14 stands and speakers are 100% compatible with the S8. The colour just don't match, as the BL14 is polished aluminium and the S8 is black coated aluminium.
So if you want to expand your system in the future and want the colour to match, then you either need to buy another S8 or be lucky to find someone selling the speakers.
The price of Beolab14 range was reduced when the S8 launched.
If you buy new I see the following options (prices in Danish KR but you get the point):
S8 and later 2 BL14 speakers: 9000 + 5000 = 14000
S8, and later another S8. 9000 + 9000 = 18000
BL14.4: 18000
BL14.2 and later 2 BL14 speakers: 14000 + 5000 = 19000
So the cheapest way to go (both in initial and final price) is S8 + 2 extra speakers. The drawback is that the colours don't match.
The most interesting option (if you got the living room to support it!) is the S8 + S8. The price is the same as the Beolab 14.4 but you get two subwoofers instead!