Yeah, I think materials-selection is the easy part. It really comes down to implementation issues of structural design, manufacturing processes and cost. A SS-321 tubeset would be about 5x more expensive than chromoly and the proprietary Gall-Tough+ stuff ends up costing 10x more expensive than chromoly. In a custom-frame like a Serotta, Eisentraut or Steelman, there's so much labor and design costs built into the frame, that an extra $400-700 isn't really gonna make much of a difference. But when you're looking at mass-produced chromoly frames in the $300-400 range, then adding that extra expense for the exotic tubing may not be worth it. You'll get a better sense of "value" by going to titanium or alloy.

Since people brought up aluminium foil, here's the properties for aluminium:

Aluminum 6061-T6
density: 0.0975 lb/in^3
ultimate-strength: 42.0 kpsi
yield-strength: 37.0 kpsi
elongation@break: 12%
modulus elasticity: 10 mpsi

Aluminum 7005-T6
density: 0.100 lb/in^3
ultimate-strength: 50.8 kpsi
yield-strength: 42.1 kpsi
elongation@break: 13%
modulus elasticity: 10 mpsi

Aluminum 7075-T6
density: 0.102 lb/in^3
ultimate-strength: 83.0 kpsi
yield-strength: 73.0 kpsi
elongation@break: 11%
modulus elasticity: 10 mpsi

The comparison metric you'd want to use here is yield strength-to-weight ratios (density) as this would let you design frames that would hold up to roughly the same amount of load before they take a permanent bend. So now we're looking at (yield-strength / density):

Yield Strength-to-Weight Ratios
184 - chromoly 4130
448 - titanium 3Al-2.5V
679 - titanium 6Al-4V
208 - stainless 321
701 - stainless Gall-Tough+
379 - aluminum 6061-T6
421 - aluminum 7005-T6
716 - aluminum 7075-T6

However, you can't design something to be as light as possible for the load, there's rigidity and stiffness to be considered as well. So let's compare the stiffness-to-weight ratio (modulus / density):

Stiffness-to-Weight Ratios
105 - chromoly 4130
89.5 - titanium 3Al-2.5V
92.6 - titanium 6Al-4V
98.6 - stainless 321
97.5 - stainless Gall-Tough+
103 - aluminum 6061-T6
100 - aluminum 7005-T6
98.0 - aluminum 7075-T6

Interesting to see that the materials are similar in stiffness-to-weight, eh? That means a frame designed from these materials to be similar in weight, assuming similar designs, will have similar stiffnesses as well. However, titanium or aluminium will end up being 2-3.5x stronger for the same stiffness as chromoly. And ordinary 304 stainless is actually weaker than chromoly for the same stiffness, 321 is similar and the exotic stuff ends up about the same as high-end titanium & alloy materials at 3.5x stronger for the same stiffness as chromoly.

As a designer, you've got to strike a balance of these two factors in addition to the design, manufacturing and costs as well.