Tsugae Reishi grows mainly in the northern forests of the world on the decaying stumps and logs of hemlock, fir and pine trees. True Reishi, or Ling Zhi (Ganoderma lucidum) grows mainly in warmer forests on decaying stumps and logs of hardwood trees. Excepting for these differences of favored climate and host trees, these mushrooms are indistinquishable on the macroscopic level.
Tsugae Reishi is often used in substitution of true Reishi, and research has shown that they share similar constituents. However, I think it is a mistake to regard Tsugae Reishi simply as a substitute. It has its own history of use in Chinese medicine, where it is known as Song Shan Ling Zhi, and is today one the major cultivated species of Ganoderma in Taiwan.
A study of three Ganoderma species, performed at the Chinese University of Hong Kong, comparing the aqueous extracts of G. lucidum, G. sinense, and G. tsugae, reported that G. tsugae was the most potent at inhibiting the proliferation of human breast cancer cells. (PMID: 17034284)
Another study from the East, has shown that G. tsugae contains water-soluble polysaccharides which produced tumor inhibition ratios of 95.1-100.0% and prolonged life span by 236.3-267.5%, when administered to mice with sarcoma 180 (Wang, G. et al. 1993).
Both of these studies are using water-based extracts from fruitbodies.