This link specifies that Alloy 20, specifically CARPENTER® alloy 20Cb-3, has an amazing resistance to chemicals containing chlorides and sulfuric, phosphoric, and nitric acids. It also resists pitting, crevice corrosion, SCC, and intergranular attack. It can be used for pickling tanks, gas scrubbers, piping, heat exchangers, pumps, shafts and valves. Synthetic rubber, pharmaceutical production and other process equipment.
While Ti 6-4 (Grade 5) is significantly stronger than commercially pure titanium while having the same stiffness and thermal properties ( excluding thermal conductivity, which is about 60% lower in Grade 5 than in CP Ti). Among its many advantages, it is heat treatable. This grade is an excellent combination of strength, corrosion resistance, weld and fabric-ability.
Comparing it to alloy C-276, specifically HASTELLOY® alloy C-276, possesses an outstanding corrosion resistance in reducing and oxidizing environments. It maintains corrosion resistance in welded condition, and is excellent resistance to pitting and stress-corrosion cracking (SCC).
It is widely used in severest environments in chemical processing, pollution control, pulp and paper.
Not too sure about the comparison with alloy 316, but the latter possesses an improved resistance to pitting by solutions containing chlorides and other halides. In addition, it provides excellent elevated temperature tensile, creep and stress - rupture strengths.
To answer your last concern, obtaining and availability of Alloy 20 that I am not too sure about.
Corrosion Resistant Materials