Definitely have to agree that Fallowfield is cheaper for shopping - not only is there the Curry Mile (i.e. lots of cheap Asian supermarkets) that you can stop off at on the way home, along with an Aldi, and a large Sainsburys two mins walk away from the accommodation. I know people in the city complain about a lack of decent supermarkets.
Plus, Fallowfield has a great social life - there are plenty of bars here (Revs, Baa Bar, Twisted, 256, Font, Wetherspoons etc.) and pretty much every society social I have ever been to have started in Fallowfield - either because we've gone to Revs there, or because the pre-drinks are at halls in Fallowfield, before we head to a club in city.
Don't worry about the cost of a bus pass - it's £185 for the year, and it covers transport throughout Greater Manchester, so if you get a job at the Trafford Centre, or join a society that requires you to go to an outlying area on a regular basis, then you don't need to worry about the cost of transport. Plus, buses down the Oxford Rd Corridor (i.e. City - Uni - Fallowfield) run 24/7, and are really very regular - they're invariably in convoy, and I've never had to wait more than 5 mins for one during the daytime.