I'm literally just copying a comment that I posted on another thread (with a few additions and style changes) because the question is exactly the same.
Literature at A-level is about analysing many prescribed poetry and drama texts, along with some unseen prose. Language and literature at GCSE both feed very well into A-level literature, as it is basically the same, except now sociohistorical context plays a far greater role (AO3), and the use of interpretations from professional literary critics is also a key element in your essays (AO5). A-level English literature prepares you well if you have any academic pursuits regarding English literature for tertiary education.
English language at A-level is very different from what it is at GCSE. In A-level language, you look at the 'theory behind language' and study the history of the English language, how it has evolved and how it has become more diverse with different people (e.g. idiolects) within different contexts (e.g. in occupation, gender etc.). You also learn about a child's language acquisition and how they develop their literacy (this spans 0-11 years). Theorists about these different aspects of language are also very significant here for both diversity and change of the language, along with its acquisition during the first eleven years alive (e.g. David Crystal, Noam Chomsky etc.). The course has an intense focus on grammar in AO1 as well, so you'd better be comfortable with it and its metalanguage (e.g. word classes, clause types, phrase types, active and passive voice, verb tenses and aspects etc.). A-level English language delves into linguistics more than anything else, and would be more suited for those with plans to study linguistics further.
There are very few, if any, transferable skills from GCSE English language to its A-level counterpart. Your ability to write creatively will only really become useful during your non-exam assessment, which has very little weighting towards the final grade. Also, if you enjoy analysing and interpreting fictional texts, then you might find literature more congenial because your skillset would be more useful there.
The workload is pretty even between the two. Both subjects have a large increase of subject terminology from GCSE, but I'd say that A-level language definitely has more. Personally, A-level language feels like more of a jump. Ask your teachers if the combined course gives a good bit of both worlds, and factor in their response when you make your final judgement.
I'm taking both at A-level, and I'd recommend one thing: choose wisely.