There really is no difference. It comes down to preference. However keep in mind that a true hardwood floor is almost always a tongue a groove product and there for only nailed on the leading edge of the board. I mention this because the wider the plank you go with the more chance it has to cup... To help eliminate this from happening be sure to allow plenty of time for the new flooring to acclimate to its new location(s) prior to installing. The rule of thumb is that the difference in moisture levels between your new hardwood floor and your existing wood subfloor, concrete, ect is with 3% of each other. Every good flooring installer will own a moisture reader to determine this difference.. Good luck

Like Jared Lewis Construction pointed out, the level of stability is what you will be choosing. A narrow plank = stable (less expansion/contraction/cupping/bowing) whereas a wide plank (5 inch is the beginning of the wide plank category) has a stronger chance of having issues with expansion.

You will need to understand your local climate, the space of installation and your HEATING/COOLING system. You will need to know if you have a humidistat attached to your heating/cooling system. If you do, you will be required to keep the temp/humidity in a very tight zone (65 - 80 F and 45-60% humidity). If you do not, then the narrower plank (with PLENTY of time to acclimate) will be the better choice.

If you are in a very dry/very humid area OR you do NOT have humidity control on your furnace/cooling system, you will want to look towards an engineered product as it is easier to work with in extreme climates/humidity.