What is the difference between the usages of bis-tris methane and bis-tris propane?
They have different acid dissociation equilibria and so buffer at different pH.

                    pKa1    pKa2
Bis-Tris methane    6.5     -
Bis-Tris propane    6.8     9


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The two titratable secondary amines of Bis-Tris propane make it usable over a wider pH range.