Well to be fair, in my experience Vodafone do have the best signal so far as range and reliability is concerned.
The reason independent phone stores are bad is because the just plain don't give impartial advice. They are paid comission, which is paid according to targets set by each network - they work to the targets which give them the best comission, then when they meet them move on the next most valuable. Sales people in independent phone stores usually know nothing about the devices or networks and will freely invent anything they can in order to get you to sign the contract you want - hence you get significantly more complaints than those who buy direct.
There is also the question of after-sales service. Buy straight from the network, and there is only ever one company responsible for providing you with assistance. Buy from an independent however, then two are, and neither will claim responsibility, therefore making it hell to sort out any potential problems.