will the wood flooring be installed above grade, below grade, or on slab?

The flooring will be installed over plywood. There is also a lower level in the house. Thank you

If the budget allows it, I'd go with wood.

Solid hardwood can be refinished many times and last for decades. Some engineered can be refinished a few times - it depends on how thick the wear layer is. Solid hardwood is usually preferred over engineered except sometimes over slabs especially in humid areas like Florida - Engineered is more stable in an unregulated humid climate than solid wood.

@jhmarie eng is not for an unregulated ( no wood is) climate because it can still expand and contract-it's for hi humid climates due to its stability(not the same) and in places like Colorado it can even dry cup. Eng has its benefits but also drawbacks as does solid flooring.

@Ultimate - thanks - I used a poor choice of words.