My final year project supervisor (I'm a final year undergrad) is offering me a fully funded Phd (with 15k pa stipend) to research machine learning in the genomic space which is super cool and interesting. He studied his Phd at Cambridge and is very strong in his field but the only downside it that I am at an ex-poly very low-rank university.

I have also been accepted at Manchester to do an Msc in AI which I think should lead to a funded phd thereafter and Manchester is one of the best uni's in the country for research in this area.

So , the question is, what the hell do I do?

I don't want to stay in academia after finishing a Phd so I need to know if a) employers will care if I have a Phd from x low rank uni rather than y high rank uni? b) if at a Phd level (people have said) its most important that you get on with the supervisor, like the topic etc and that uni prestige is meaningless at that level ?

Dont mess around worrying about the so-called ranking of universities. Companies worth working for dont care where you studied.
What matters is what you achieve during your phd, your relationship with your supervisor and that your university has the resources you need to succeed - computational in your case I think. Leave all the worrying about university rankings to schoolchildren and parents who dont know any better and are dazzled by such nonsense as league tables.

Couldn't agree more. Take the PhD. Rankings at PhD level are even more meaningless than usual. Plus, to coin a phrase, a bird in the hand is worth two in the bush.

It really is incredible how many people buy into this league table rubbish. I keep hearing from people who talk about being at a Russell Group uni as though their presence there has somehow impregnated them with special magical powers. I never know whether to laugh or cry at this.

I think the way to break this problem might be to do something as drastic as ensuring there is a common curriculum and a common set of exams across all universities in the UK. When you are charging students just under £10k a year, there needs to be some meaningful way for them to know what they are buying. Right now the current system encourages and rewards corruption and bullshitting.

Agreed. Some of the teenagers in my family are looking at Unis, and every one I have suggested (as it is near a support network/family, or a really nice place or I've heard very good things about a department) they have shut down immediately as "it's not RGroup and I won't get hired at the end!"

I went to an ex-poly for my undergrad and I still managed to get a funded PhD in a very well respected department.

I disagree. ive recently seen job applications which specifically ask for a degree from a RGroup universities. I think this is going to become more prominent with there currently being a movement in the UK away from the idea that everyone should go to university even if its to a low ranking university to do a pointless degree. Also speaking to lecturers I have heard many saying privately they look at PhDs from a well ranked university better than one from one lower down.

Job applications for where? I don't doubt that City finance companies, for instance, filter by that metric, but it doesn't sound like that's what the OP wants. Also, it sounds like you're talking about bachelors degrees which are a slightly different ball game.

Regarding PhDs, what 'ranking' are we talking about - Guardian/THE tables or departments that have well-recognized research capabilities in a particular field and strong supervisors? Because at PhD level the former becomes far less important than the latter.

I wouldn't bother with a PhD at a post-92 university that didn't offer excellent supervision and a stellar research track record in my field, but neither would I bother with a PhD at a Russell group that didn't offer the same. RGs, by dint of their deep pockets, attractive reputations, and well-resourced departments may be more likely to offer that, but being a Russell Group university alone doesn't automatically mean a better PhD experience or better prospects at the end of it. If some lecturers don't grasp that, then they are misguided.

The number of job adverts specifying an RG university will be vanishingly small.
Same goes for the view of your lecturers. The reality is that most PhD positions attract a relatively small number of applications. Supervisors on the whole don't have the luxury of simply turning people down because of where they did their undergrad degree.

What you are witnessing here is a pocket of stupidity and snobbery in industry and academia which deeply hurts both. Some of the biggest companies in the world have even stopped asking for degrees altogether for some jobs because they know that only asking for Oxbridge grads seriously harms their business.

You mention above that with more people getting degrees that employers are likely to ask for more RGT graduates as a result. In fact this is not what employers do in practice. What they seek is experience - both life and work. You need a great degree from a recognised university and experience which makes you stand out if you ever harbour hopes of getting into the biggest companies.

I don't know who is advising undergraduates about choosing RG unis because of improved job prospects but they are doing young people a serious disservice when they do so.
Go to a recognised university, get a 1st class in your subject and secure some work experience directly related to the job you want even offering your services for free if needed. That all matters much more than coming from an RG uni.