I chose a shop first, and went from there.

I went into a few shops and told them what I wanted -- a road bike for doing long rides, so it had to fit me well, with a specific budget in mind. I chose the shop that gave the answer I liked best, which was that they would first measure me using their fit kit, then would recommend a couple of bikes in my budget range. At the time, that shop sold Treks and Giants, and I wound up with a Trek. We've made many adjustments in the fit since then (this was in 2002) but overall I've been happy with the bike. Most important, I'm very happy with their service.