I think that the higher degree classification would be more highly regarded.
In terms of which language, there are a number of ways of looking at it:
Firstly, what do you hope to do careerwise? If you hope to go into teaching for example, then definitely choose Spanish and Portuguese as these are more widely taught in schools ; there isn't an exam board which offers GCSE or A Level qualifications in Korean.
Secondly, Spanish and Portuguese are more widely spoken than Korean, therefore studying these will provide you with greater opportunities in a wider variety of places.
On the other hand, there is a lack of Korean speakers in the UK, meaning that you will have a skill that not many people have. Having said this, if you complete your first year at SOAS, you will have a basic/intermediate knowledge of Korean, which you could possibly use later on.