Birch gives the most Btu per cord at more than 23 million. Cottonwood only produces about 14 million Btu. White spruce checks in at 18 million Btu and black spruce at almost 16 million.
If you can get a cord of birch at the same cost as a cord of cottonwood, assuming they are at the same stage of dryness, birch is the better buy.
Cooperative Extension has an excellent website with information on burning wood. It can be accessed at www.alaskawoodheating.com. There is a calculator to help you figure out your heating costs as well as lots of information on how to get the most from your wood.