For the most part I make my decisions based on the content posted and or curated by the witness, there level of community activity and community reputation. There are some Witnesses like @ausbitbank who put out reports on their activity. This is a very good sign and provides all the info you need to make an informed decision. Admittedly there were a couple i picked just cause I like their name. It should be noted the choices are not permanent. You can change who you vote for anytime as often as you like.