The two parks are actually quite different. The focus at Sequoia is the big Sequoia trees. These trees are just awe-inspiring. They are so huge and the oldest are several thousand years old. Every time I see them, I'm reminding of just how insignificant I am in the scheme of things.
Kings Canyon is less crowded. I believe it does have some Sequoia groves at the entrance to the canyon, but not like Sequoia. It's more about the canyon scenery, the river, etc. It takes awhile to drive between them so if you want to visit both, I'd say to split your time evenly. They are both worth a few days. Both have good long and short hikes. The ranger stations will have info on those.