'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In very informal (and rather outdated) British English, 'not half' is an intensifier.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':
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.
October 21, 2019
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