Is law or business and law better to do at uni (what one results in more money)

Law or joint business and law

I particularly hate these sort of questions, but at least it's not vague.

Your degree does not result you in getting more or less money. Your employment/enterprise results you getting less or more money.
A degree is not a passport to instantly getting more money. A degree (or any qualification for that matter) does not instantly entitle you to getting more money.
Getting a super respected degree with top grades does not mean you can't be broke and be living on the streets unfortunately.

Irrespective of your qualifications, you would still need to be able to add value to companies, communicate that value, and be able to work hard. Getting qualifications doesn't guarantee that, and neither would it guarantee your employment/business i.e. it depends on what you bring to the table, not what certificates you have plastered on your wall or how many letters you have after your name.

To partly answer your question though:
If your degree is an LLB, then you would be able to apply for roles as a barrister without further degrees. This applies if the degree is in straight law or law and business. If it's a BA, then you would need to do a PGDL in order to become a barrister.
Law firms also tend to prefer the LLB over othe degrees for their SQE training contracts from what I have heard, and if you don't have an LLB you would be asked to do a PGDL for the solicitor roles prior to doing the SQE (even if it's not required).
The joint business degree would allow you to go into business research in academia and provide you with a foundational knowledge in business. However, you can obtain the same necessary business knowledge outside of uni e.g. Business Studies A Level.
Otherwise, you can do a job that asks for a degree in any subject or require no qualifications at all with either degrees.

In my book, so long both degrees are LLBs, it doesn't matter which one you take. I would then just pick the one that you enjoy the most.
If the law firm/employer that you want to work at is picky about which university that you studied at, then I would be a bit more wary about the choice in course.