Hi-tensile steel is both stronger and more durable than aluminum. However, after being properly designed, engineered, and constructed as a bicycle, aluminum can be made to be almost as strong, but not quite as durable. Almost all modern-day bicycle frames are made to be quite strong and durable.

Read about what the bicycle guru, Sheldon Brown has to say about bicycle frame materials and their construction below:

http://sheldonbrown.com/frame-materials.html

That said, chromoly steel is both stronger and more durable than hi-tensile steel.