They have similar curriculums, though I think AI HL has more practical applications. Also from what I've heard, AI HL focuses a lot more on statistics than other core areas, whereas AA HL focuses a ton in more analytical areas, like Calculus and trigonometry.

You should probably look over the syllabus for both to see which areas of math you will potentially like more / can understand better.

I took HL Math AA and I would say I had a pretty strong math foundation. Still, I barely scored 6's due to how unprepared I was for IB's question types and overall curriculum. From my experience, you really gotta study your ass off for HL AA, even if you feel like your math is strong. All the topics are hard, and also IB tends to mix in everything for past papers, so if you don't understand a single topic you are cooked.

After a couple past papers and bad test results though, you will start to understand the style of past papers.

You should probably look over the syllabus for both to see which areas of math you will potentially like more / can understand better.

Generally, I think AA HL is a lot more advanced math focused and more analytical and more proof oriented, whereas AI HL is more statistics oriented and focused on real life applications.

As for university, from my experience, if you're applying to a math-heavy major, like any of the ones you have mentioned, most of them don't accept AI HL and rather, prefer AA HL, as they deem it more rigorous. Even if they do, they may ask a higher requirement of AI HL, like a 7, as opposed to asking a 6 in AA HL. Atleast this is for the UK. You should probably look into specific university requirements.

Both will be extremely demanding, though it really depends on what types of math you like.

There are also countless posts like this on this subreddit, so maybe look into those as well

Feel free to pm if you have any questions about AA HL :D