That's easy. Taiji is a martial art so its principal energy is Spirit, whereas Qigong is a class of energy exercises, and its principal energy is Qi.
The next question is always: What's the difference between Spirit and Qi?
Spirit is the vital animating spark that brings things alive or wakes things up, whereas Qi is a pervasive energy prevalent in Nature, especially in Living Nature.
Life requires both but Spirit precedes Qi.
When walking in Nature, experiencing its beauty and vibrancy then you are feeling Qi. If you are suddenly startled by a noise in the undergrowth and your hackles rise and your senses sharpen then you are experiencing Spirit.