Rita hit it right on the head. Go to a store with a wide selection of brands, models, price points, and features. What Rita would tell you is her ideal treadmill is probably different than what I would tell you is my ideal treadmill, simply due to differences in our running. Finding a store with the widest selection possible will allow you to try them out and get a feeling of what you need, what would be nice, and what is just a waste of money.
The store Rita went to sounds like a great place, actually suggesting to her that she sleep on it, really think the decision through, and even try the different models more than once. If you can find a store that will give you that kind of service and will have a wide variety of offerings, you will be a satisfied customer.