All species of wood contain roughly the same amount of creosote per pound of wood (not wood plus water, just wood), and roughly the same amount of BTU per pound of wood. The hardwoods are more dense and contain more actual wood per unit of volume than a soft wood does. Thus a hardwood will be heavier, and burn hotter than a soft wood.
The difference comes in the temperature of the air as it rise out of your chimney. When the wood burns, the creosote is a by product that is released into the air. So long as the air stays hot, the creosote will stay airborne. If the air cools, the creosote will start to deposit on what ever it can. So if your chimney stays hot enough, it will have very little creosote build up in it. However, if your fire gets too cool and the gasses in the chimney cool down too much, all of the creosote will be deposited in your chimney and that makes for a dangerous situation in a hurry. Since soft woods do not contain as much mass per volume, it can be more difficult to keep the temperatures up where you want them. Same goes for "green" wood. The wood has to be dry to burn, and when a piece of green wood is added to the fire, part of the heat of the fire is used to evaporate the moisture. That, in turn, loweres the temperature of your exhaust gasses and allows for more creosote build up.
The bottom line is that if you keep the exhaust gasses hot enough, you will have a minimal amount of creosote to deal with, but if you don't, it will start building up on you faster than you may like. The cooler the fire, the more smoke you get, and the more creosote build up. When you add a fresh log to the fire (even a seasoned one), you will get a little smoke, and a little creosote because a large portion of the heat from the fire is being used to ignight the new log. The new log will have SOME moisture that needs to be burned off. Once the log up to temp, the smoke should deminish the temps should rise, and you should once again have very little creosote building up in your chimney.
