IB places English in two different groups, and the two courses have different purposes.
English A
Studies in language and literature. The assumption is that the student already uses English fluently; the course is about analysing texts, not about learning the language.
It comes in two variants: Literature, which works only with literary texts, and Language and Literature, which adds non literary material such as advertising, journalism and speeches.
English B
Language acquisition. The purpose is to build the language: reading, writing, listening and speaking, around a set of themes.
HL goes further than SL and includes literary works, which surprises students who choose it expecting a purely practical course.
Which is right
The honest test is whether the student can read a novel in English and write an analytical essay about it without the language itself being the obstacle. If yes, A. If the language is still the obstacle, B.
Taking A when B is appropriate produces a low grade in a core subject. Taking B when A is appropriate produces an easy grade that some universities read as weaker evidence.
The university requirement comes first
This is the point most often missed. Many universities accept English A at a stated grade as proof of language proficiency, and treat English B differently or not at all.
That means the choice can decide whether a separate proficiency exam will be needed. The requirement is published on each university's own pages and should be read before the subject choice is made, not after.
The diploma rules
The diploma requires a language from the studies in language and literature group. English B alone does not satisfy that requirement unless another language A is taken.