On the question of how to choose, I would start with a basic question about the frame, since that is the one part you really can't swap out: do you want a diamond, step through, or mixte frame? If you will be riding in skirts or dresses or if you will have children on the back of the bike, than I recommend a step through or mixte.
After you settle on the kind of frame, then I would ask whether you want internal gears or derailleur gearing and rim or hub based brakes. If most of your riding will be short distance, stop-and-go, relatively flat city miles, then I'd recommend an IGH. If you plan to ride in all weather -- rain or shine -- then think about hub brakes (drum, roller, or disc).
After these decisions, then you can start thinking about specific brands and models. I've just recently been through this myself and ended up with two bikes: one for commuting and another for long, recreational weekend rides.