You should chose your witness based on the following:
- how many missed blocks they have (when they machine didn't validate blocks losing all the transactions in it, happens)
- how often they are updating their witness thread (my initial post is here: https://steemit.com/witness-category/@dragosroua/dragosroua-witness-thread) and I update my status every Monday
- how involved are they with the community
You can see some of these props (missed blocks, ranking, price feed) in any block explorer, but I recommend this: https://steemd.com/witnesses
Steem on!