You've seemingly reduced 'Delta is more transmissible' to its most meaningless interpretation.

A large (mostly) unvaccinated and uninfected population has been exposed to both variants and the delta variant won out, what more proof do you want?

Delta will spread faster. We know this from seeing it spread in countries that have low vaccination rates and easily outcompete other variants. This can also be seen in laboratory experiments.

Delta since it outcompeted Alpha everywhere in the world.