Hi, I am doing Advanced Higher Spanish this year and I'm really enjoying it (just for context I am planning to study languages at university next year). One of the main differences from Higher to Advanced Higher is you will write a portfolio essay on some literature (could be a film, play, novel or short stories etc). The essays are about the topics you are studying rather than a past tense essay. The speaking exam won't have prepared questions as such and it's an external examiner. The reading is quite similar, just at a harder level with a longer translation and an overall purpose question which is like an extended inferential question. The listening still has a monologue and a conversation just at a harder level and each extract is only played twice, with the writing and listening in the same paper (so you actually have a dictionary in the listening). The topics you study will be more in depth, and you might have a chance to vote on what you'd like to study (depends on the teacher).
I'm not sure what the requirements for computing at university would be but it may be you have already met them if they are just based on Highers. I think a language is always a good subject to have but of course I am biased as I love languages! I'd recommend you check out university requirements/preferred subjects and ask your teacher, but hopefully this is helpful in giving you an idea of what Advanced Spanish is like!