Hi Pongo,
It really depends on a number of things. You say Coventry is better ranked and you can live cheaply. Great! If you can avoid putting undue financial pressure on yourself during your studies then DO IT. That bears repeating: DO IT!! Salford may be the more attractive option for you in terms of fun, but if your course is short (I'm assuming it's a masters since you mention undergrad independence), I personally think it's not worth risking your financial security for independence you can easily have once you graduate and are working. Manchester's not going anywhere! Besides, if you haven't done postgrad work before, it's easy to underestimate the amount of time you'll need to spend studying. Then it absolutely sucks to be in a city full of fun stuff you can't do because you're chained to the books!
If you're weighing up PhD options, why not try to compromise? Do the better course in the cheaper city, which I think is probably Coventry, and move into an affordable house share much closer to campus. Then you'll get your independence, spend less money than you would at Salford, and still have the easy option of going back to mum and dad's for Sunday lunch / emergency respite if you need it!
With Coventry, the commute is manageable if: a) your need to be on campus is limited and you can maximise library time while there, and b) you can use public transport, treat yourself to some noise cancelling headphones and spend the commute studying. If you need to be in a lab 24/7 for example, then whatever your course you need to live a little closer to the campus, I think.