If you were looking for a personal trainer what sort of person would you choose? If you were looking for a driving instructor what sort of person would you choose? The key is finding someone with whom you can communicate and feel comfortable. You're going to be spending many hours 2-300 feet up in the air in a cosy setting doing something technical. I had an unpleasant experience at Stapleford with a snarky individual who seemed to almost take delight in belittling me and trying to turn me into an aerobatic pilot - something I never wanted to be. He was kind of like the school sports coach who everyone hated. When it became clear this wasn't working I looked elsewhere and found someone completely different - someone who was professional, approachable, relaxed and genuinely interested in flying and helping people fly. He could explain something in about 10 seconds that the other bloke would have taken weeks over.
Stapleford has quite an idiosyncratic culture and the instructors / admin staff are either really nice or really surly. It's in for a penny, in for a pound with them! If you're going for a PA28 then G-SACO is their best plane. The other one, SACI, reeks of WD40 and has a manky tube of softmints stuck in the seat. I quite liked Stapleford. I miss the tea and crumpets served in the clubhouse and some of the staff were lovely. I miss the hares on the runway and the unique ambiance!