Gray-cheeked Thrushes breed in northern Canada, Alaska, and even Siberia as the forest gives way to tundra. These birds prefer thickets and stunted spruces with dense undergrowth. Swainson's Thrushes breed further south across Canada and Alaska, south into the Rocky Mountains (all the way to New Mexico) and northern New England. They are found in coniferous forests.
Where their ranges overlap, these two thrushes may hybridize (Mack and Wong 2000), which can complicate identification. Gray-cheeks can show quite a bit of white around their eyes. The white can look buffy in difficult light. The buff on a Swainson's can be difficult to see in poor light. Some races of Swainson's Thrush are quite gray, while others are remarkably rufous. Hybrids, if they actaully exist, only further complicate identification. My understanding is that a hybrids are hypothetical. Discovering a hybrid for sure will take a graduate student immune to blackflies.