In service terms, I have personally found npower to be v.poor, southern electric good enough, british gas hit and miss. If price is a bigger driver, bear in mind quotes include cost over a year, but with some providers (e.g. npower) you need to be supplied by them for the whole year to get their discount, wheras others (e.g. BG) you get it quarterly. npowrs picing historically pulled you in then slapped the prices up, so moving before a year was up was necessary for me. hence they were much more expensive than quoted.

if there is anyone out there with good customer service, i'd be interested to hear about it!