What each one does
Soy gels more strongly and emulsifies more efficiently at the same protein level, and it is the better functional material. Pea avoids the soy allergen declaration and the market resistance that goes with it, and reads better on a clean label.
What decides it
Whether the constraint is performance or positioning. If the product must hit a firmness target at a given cost, soy. If the brief says no soy, the formulation has to be rebuilt rather than substituted.
Where the obvious answer is wrong
The obvious answer is to swap one for the other at equal protein. The product comes out softer and tastes earthy, and the fix is not more pea protein but a different texture system around it.
