You’ll find a lot of argument on this topic, but it really depends on what you are looking for from your fire. Most firewood dealers sell (& recommend) mixed hardwoods.   In general, softwoods light and burn easily and quickly with a hot fire which tends to make a lot of sparks (and even exploding sap pockets). Hardwoods are usually harder to start but burn more evenly and quite a bit longer, with less smoke/sparks.  Softwoods (like Pine) have the advantage of being readily available/renewable & season more quickly.  But, firewood is usually sold by volume and heat production is dependent on weight. Pound for pound, all wood has approximately the same BTU (heat) content, but a cord of seasoned hardwood weighs about twice as much as the same volume of softwood, and consequently contains almost twice as much potential heat. In my opinion, softwoods make the best kindling & hardwoods the best firewood.  If the wood you are buying is not all hardwood, consider offering a little less in payment.