How important is a large platform? Or does the stiffness of the shoe matter as much or more?

I am considering getting new pedals, been riding/racing on Speedplay Frogs (I know you are cringing.) and like the weight, longevity, and the fact that I can commute easily on my race bike to work with mtb. shoes.
I use carbon soled road shoes when I am not commuting. Keywins have caught my eye but are expensive to get the same weight as the frogs.

Thanks.