PROs and CONS:
Here are some things to consider...a softer wheel will be really grippy, but will roll a little slower. A harder wheel will roll really fast, but will lack somewhat in grip. Remember it is all personal preference, what works for someone else might not work for you. Also keep in mind that floor and surface conditions constantly change, due to weather, cleaning schedule, wear and tear, etc. Buy a few different sets, or trade with a friend to see what works for you. The most common set up is to have a mix of 4 and 4. One set of 4 on the soft end of the spectrum, and 4 on the harder side. Experiment with your wheels and have fun.