At least for now, the vintage bikes that are climbing in value are the steel bikes. So if you want a bike you can enjoy for many years, and one that will appreciate in value, steel is the choice. Perhaps part of the reason is that there isn't much modern steel out there. In the 1980s, almost the entire Trek product road bike line was steel. Now they sell one steel road bike. And people just like the look of a lugged steel frame.