I think that frame materials are difficult to discuss intelligently on a board like this because the topic reaches out in too many directions.

First of all, you don't ride on Cro-Moly or Hi-Ten steel. What you ride on is a bicycle frame that has been made from those materials. A designer is likely to start at what he wants his finished product to be and work backward to the raw material. Gary Bontrager's mantra is "Strong, light, cheap. Pick any two."

Different metals have differing tensile strength. If you take a rod of metal with a 1 sq/in section, tensile strength is the amount of hanging weight it will support before breaking. I don't know the numbers off the top of my head, but steel compares pretty good with other materials. All steel alloys won't be the same. Cro-Moly will be stronger than Hi-Ten and way stronger than aluminum.

You can still make a good frame with Hi-Ten, you just have to use more of it. That's obviously going to make it weigh more.

If you are making a frame from a material, like aluminum, that you have to use a lot of to get the tensile strength that you need. You can cheat a little by playing with tubeing sizes. A big tube, if everything else is equal, will be harder to bend than a little tube. Since you have to use more aluminum, you can draw the amount of material that you have into a bigger section tube. The limit is determined by how thinly you can draw the tubeing walls without making them too subject to physical damage (called beer canning). Steel frames that are alloyed with exotic materials (niobium and vanadium) have tubeing wall thicknesses on the verge of beer canning.

So far as joining methods go, just let me say that the material used will dictate the joining method. The companies that draw the more exotic bicycle tube sets will specify how they have to be joined and sometimes require the frame builder to submit qualifying samples before they will sell them tubeing. Several years ago Santana, a company which specializes in tandems, made a run of Nivacrom steel framed mountain bikes. More recently, they produced a run of very light Scandium aluminum road bikes. Those projects were undertaken so that Santana could gain experience working with those exotic materials before having a special run of tandem tube sets produced.