While one can generalize about the relative hardness of wood, in reality it depends on many factors:
--where did the tree grow? shorter northern growing seasons usually result in denser wood.
--how did the tree grow? hillside, meadow, dense forest, etc.
--what minerals were in the soil? this can have a small effect on hardness.
--what part of the tree were the planks cut from? stump wood, crotchwood, sapwood, etc. I assume they are slab sawn.
Only you can determine how hard they are. One of the planks may be better than the others.