Z5 is a thicker formula that fills swirls better, and faster. They both have a different look to them, and some people only use Z5 and prefer its shine. Some people prefer Z2. I like Z2 on darker cars and Z5 on lighter cars, although they both work great.
In the lab, Z2 has higher optics, but according to your color car and your eyes, finish up with whatever makes you happy and you feel looks the best.
Make sure you use Z6 in between coats...it makes a difference.