In jackets and pants, the debate rages as to whether Kevlar trumps leather. And the same is true of whether you should reinforce bones with steel alloy inserts, or whether you should just pad them and allow your flapping limbs to absorb shock that could otherwise damage more important things, like your spine and head.
With boots, there's no question: Big, bad plastic trumps leather. Just look at the difference between racing boots and street boots -- high-speed racing boots almost universally include plastic or composite shells for sliding across the pavement, and a more supple liner to keep your feet comfortable.
You don't need your boots to be supple and flexible all the way around. You need them to take a licking and keep your feet and ankles pointing in the right direction. Go big. Go bad. Go high-tech composites and plastic.