Bottom line......monthly cost!!! Investing is primarily about making money, so how the criteria is set is (for me) set by the same standard. I opt for a competitive (but fair) price negotiated at a discounted rate for multiple listings. I don't necessarily believe in the "cheapest" price because that is not a guarantee of good service, but I ALWAYS negotiate a fee that is agreeable to all parties.
Sure you can go on the recommendation of others, but just because they are doing a good job with your mate's rental(s) doesn't mean they will perform equally so in looking after your interests. At the same time, I wouldn't discard the opinion of those I respect, I just wouldn't take their word as gospel because their needs may differ from my own. Keeping an open mind works well in this instance!!
As for the other factors, how the property manager dresses, speaks, conveys him/herself, or the impressive and copious amounts of money spent on advertising campaigns (ie. one company with a long history in RE that promotes the "red carpet treatment" does the complete opposite in my books) means jack once you have handed over the keys!!!
Nonetheless, irrespective of how you select your PMs (the yardstick you use to measure them by) at the end of the day, if they don't stack up, then take (back) your keys and your business to someone who does!!!
Do your homework....you researched prior to buying the property didn't you, so why stop now???