Dragon ball z or dragon ball Kai?

First of all, I am confused as to why this is a debate, so if someone could explain that I would be very grateful. My boyfriend’s taking me through dragon ball, we have about 3 episodes left and I heard him and his brother debating whether or not I should watch Kai or Z next. Are there different characters? Why is this like this? And also, which one do you all think I should watch first?