This issue is not soft or hardness per se, but how much deformation between the material plastically yielding (yield strength), and complete breaking (ultimate tensile strength). To determine this for materials, they take a known spindle size of material like pencil size, and yank on it with a genuine Instron(tm) tensile test machine (I've never seen a different brand of these, they were always Instron), and it prints out a graph of properties.

Stronger alloys of steel or aluminum, tend to have less plastic deformation before ultimate failure. However, with a higher yield stress, they can flex elastically further, without yielding. Like materials used for springs. Generally, aluminum does not like flexing, it causes fatigue failure (see below), which is why aluminum frames (like my old Cannondale) tend to be very stiff (highest measurement at the time on Bicycle magazine's tarantula(?) test rig), great to put the power down but rode harsh, while traditional steel frames had more flex when hammering the pedals in a sprint, but better ride quality.

But if you want "graceful degradation" as failure mode, you want lower yield strength with a good spread between that and the ultimate tensile strength.

Fatigue failure is different. Lots of cyclic stress below the yield strength, seems like zero damage, until suddenly cracks. Raising the yield strength, generally improves fatigue strength. Generally, steel (and titanium) tolerates fatigue stress, better than aluminum. With steel, below a certain stress level (fatigue limit), it has infinite fatigue life. Titanium has excellent fatigue life. Aluminum has no fatigue limit, so eventually it will fail, however you engineer the bike so that it takes 50 or 100 years of hard riding until that happens.

Not sure if my posts haven't been going on.

I bought my wife a Litespeed Arenberg frame to build up with these bike parts.

I figure that you'all gave me solid advice, and I'll follow it.