It comes down to the precise math - the temperature you hold your stout, and the pressure you use for the beer gas - that would determine exactly which blend would be the best.
I'm using 75/25 these days and if you go back to
@doug293cz's post
here with my temperature and pressure combination that works out to around 1.2-ish volumes of CO2 - close enough to what I try to carbonate the stout to before putting it on beer gas. Otoh, if you're going to keep your stout warmer but use the same 35 psi pressure, you may want to go with the 70/30, so the higher CO2 content will counter the warmer temperature...
Cheers!