Service, quality and price in that order. I used city and their service is dire. I used Plumb centre and they have all got serious attitude problems, well in my area anyhow! I now use independants and they cater for my above list. I also find independants are a lot more flexible on price as they don't have a management charging head office to keep sweet as well. I find that befriending the counter staff is a big bonus, and I always chuck a fifty in their xmas fund pot. But I am biased as I used to manage an independant. Thats my 2p spent.
extra note: Just because you get a good price on your end feed ftgs, does not mean you get a good price on everything, Have a chat with the manager and get some terms set up on your account, I'm not teaching any of you to suck eggs but almost stating the obvious. But from my experience many plumbers don't even check as they pass on cost to customers, but remember screwfix is now almost everywhere and customers/people check prices with them regardless of the dubious quality of some items. Damn thats 4p spent now.
