I imagine that when you go in there somebody can help you with this, and that somebody will probably have a good idea of what's available because I bet it's pretty random.
Comfortable to ride, easy to pedal in the same gear ratio. There's no better benchmark of a bike than that. Comfortable means it fits. Easy to pedal means the drivetrain is doing its job and the frame is good. But it can be hard to compare a bunch of bikes in the same gear ratio without pulling out your slide rule. Especially with older stuff.