Hardness: The Goldilocks Zone
Wood hardness is measured in what's called the Janka Hardness Scale. The higher the number, the more resistant to cuts and dents the wood will be, but conversely, the higher the number, the harder the wood is on fine knife blades. Like many things in life, the harness of a cutting board is a balance. The harder the wood, the longer it will last, but at the sacrifice of more frequent knife sharpening. Softer cutting board wood will wear out faster, but your knife blades will stay sharp longer.
Sugar maple - Hardness 1450
Oak - Hardness 1220
Cherry - Hardness 950
Ash - Hardness 1320
Alder - Hardness - 590 (Too soft)
Poplar - 540 (Too soft)
Hickory - 1550 (Too hard, and may have some extractive problems).