I get your frustration at the lack of responses, but to receive, sometimes you have to put something into the discussion. For example, you don't mention whether you are interested in the BSc or BA programmes and what kind of outcomes you are seeking from your degree and how important the City experience is to you vis-à-vis academic reputation. There is a massive difference in terms of difficulty and prestige between BSc and BA, with the former being far harder, better regarded by high-paying employers and much more like an applied maths degree than the kind of economics you would study at A level. In my opinion, this is a far bigger choice than which university or city.
Given that the medium-term employment outlook is pretty pessimistic (state of the UK economy, Trump and AI rollout), the odds that either university provides a realistic pathway to elite-level jobs (i.e. investment banking) is diminishing, especially so if you are focusing on the BA route. In my perception, both universities are probably on a par, along with Nottingham, but below Oxbridge, LSE, Warwick and UCL and above the next tier (e.g. Leeds, Birmingham, Exeter, etc). Therefore, I would place more emphasis on the quality of the university experience, which is largely going to be governed by the City and campus life, which is clearly a personal choice, but you can't go wrong with either. Even more important is the quality of personal relationships you will make at university, which can be lifelong. If you are really fixated on a high-paying career, I would suggest that the quality of your extra-curricular activities is probably more of a differentiator than which of these universities outside of the really elite tier.
By far the best poster on Student Room on this specific subject is Ben Ryan and I would urge you to search for his posts and review his opinions, as I think he is pretty spot on.
Good luck and don't fret too much. You will have a great time at either Bristol or Manchester and concentrate on enjoying the experience rather than worrying about relative rankings.