If possible, sticking a digital antenna on a roof is the best plan. Why? Because a digital signal is either "on" or "off." The old days of being able to get by with a very weak, analog signal are over. Signals that were too weak would produce a somewhat snowy or fuzzy picture on analog sets, but a weak digital signal won't be displayed by a TV set at all.
Oh, and those roof antennas that you used with your analog TV? They'll usually work just fine with your HD set and provide strong signals. However, you have to make sure you have a digital tuner that can receive those signals (most HD sets have them built in, whereas adapters must be purchased for analog sets).