I want good customer service from intelligent and engaged staff, and easy resolution if there are any problems. So John Lewis or Apple for me. Currys have poor after-sales service in my experience, don't know about PC World.
Talking face to face with the chap who does nothing but build and fix computers (and will build yours) simply won't happen if you go to any of those places; more likely that their sales staff will be trying to sell you what the manager tells them to shift this month! Aren't Currys and PC World the same company?
This is why I recommended finding a small specialist outlet.