'B' is better than 'A'. In very informal (and rather outdated) British English, 'not half' is an intensifier. So whereas 'not bad' just means OK ( so-so), 'not half bad' would mean 'really good'. Here's what the Cambridge Dictionary has to say about 'not half': https://dictionary.cambridge.org/dictionary/english/not-half (UK informal) used in spoken English to express a positive statement more strongly: It wasn't half crowded in the club last night (= it was very crowded). She didn't half shout at him (= she shouted a lot at him). "You enjoyed yourself last night, didn't you?" "Not half !" (= Very much!) Should you use this expression? No. Definitely not.