Sevens and Eights are both aggressive types (PT, 433-36) and can resemble each other in certain respects. Both are powerful personalities who are able to go after what they want in life, but what they want, and how they attempt to get it, are different.
Sevens are primarily interested in variety–they want to sample as many different experiences as possible and become practical in as much as their practicality gives them the means to pursue the experiences they want to try.
Eights, by contrast, are more interested in intensity–they care less about variety than about having intense experiences that they enjoy. Eights are also interested in power, both as a way to maintain their independence and as a way of asserting their dominance in the environment. Sevens are not particularly interested in having power, seeing the work necessary to maintain it as possibly infringing on their freedom.
Eights are an Instinctive type, and as such, make decisions from their "gut" instincts. They prefer dealing with practical matters, and although emotionally volatile at times, generally remain grounded and down to earth. Sevens are Thinking types, and can have brilliant, quick minds. At the same time, Sevens can get ahead of themselves with their plans, schemes, and interests: they can have trouble staying grounded and on track with their projects. Sevens see themselves as idealistic optimists, while Eights see themselves as hard-nosed realists. Compare Sevens Mike Myers and Goldie Hawn with Eights Danny DeVito and Roseanne Barr.