Bristol definitely isn't in the countryside - there's 500,000 people living there! On the plus side though, you will get to walk across 400 acres of open common land (the Downs, which confusingly are on top of a hill. According to my grandpa, they were so named to confuse the Germans, but that's not actually true!) on your way to uni if you live in Stoke Bishop halls.
When you refer to hostels, I think you actually mean halls of residence. Hostels refer to short term accommodation for people who are travelling through (usually backpackers on a very tight budget), and have dormitories e.g. those operated by the YHA. They are often referred to as "backpackers hostels". The term can also be used to mean accommodation for the homeless ("homeless hostels") and people recently released from prison ("bail hostels"). On the other hand, halls of residence (often referred to just as "halls") means university-owned accommodation, specifically for students. Students will stay there for a full academic year, and have their own bedroom (and sometimes bathroom), but share other facilities such as a kitchen.
To be honest, I don't really know anything about UoB's halls of residence, because (whilst I did live in Bristol for 19 years), I was never a university student there, and I never visited the halls. I'd suggest asking on the Bristol forum for more information.
With regards to Manchester's halls of residence, arguably the best accommodation lies in Fallowfield, and the distance from uni (2 miles) shouldn't really worry you - it'll take you a total of around 20 minutes, from leaving your front door in Fallowfield to walking into your lecture, if you take the bus, and a bit less if you cycle. Don't make the mistake of thinking that you will spend 0 time getting to uni if you live in city halls - you could easily spend 20 minutes walking from some city halls to the buildings where you have your lectures. However, this is where international students are concentrated, so you may prefer this.
Fallowfield is the main hub of student social activity (with the exception of some clubs, which are also a fairly long distance away from some city halls). It also has a big Sainsburys (supermarket), whereas people who live further into the city are stuck with small, overpriced shops. Personally I think en suite isn't worth it, and catered is expensive, and the food grim. Your only option if you want Fallowfield + self catered + shared bathroom is Oak House, where I live, and I would recommend it.