Which is better, aluminum or steel?
Well, there are advantages to both, but most people will appreciate aluminum over steel.
The main reason for this is that aluminum is stronger by weight, so you can get the same durability while saving on the weight. But this does not mean that aluminum frames are stronger than steel. It just means that for roughly the same level of durability, the aluminum bike will weigh less.
Aluminum is stiffer, too, and this is where the debate comes in. It can be hard to detect if you don't ride a lot, but steel does have a slightly softer feel to the ride, which can help when you are putting in the miles. If you're riding less than a couple hundred miles a week, the effect is negligible. Steel is also the original, classic metal for bicycle frames. Many riders enjoy the sense of heritage that they get when riding an older steel frame or one made as an homage to vintage models. The brazed lugs on these bikes are also admired for their beauty and craftsmanship, and are much more artful than the welds that bind an aluminum frame together.
All in all, though, aluminum is the pragmatic choice for a great, light hybrid bicycle.