Make certain that you order the 820 in your correct frame size, which may be the 19.5, or more likely, the 21". This is where your LBS Trek dealer may be able to help you. Explain the free bike offer and ask them if they'll help you determine which size to receive. Best payoff is starting with something that fits your reach and riding style. Otherwise, like many of the other posters, I'd say just ride the 820 for a few years until you know what you'd like to ride and why.