I'd always pick Preston over Hancock, below his eagerness to help every single person in the Commonwealth he's really nice. Hancock's just a drug addict and he'll give you radiation burns. I don't have enough Rad-Away for that ...
Well, Hancock is also a person who would be very eager to help anyone in need. He's also nice, but he just has a different way of dealing with the bad guy than than Hancock. Although, if I want to go for a goody two shoes, I go for boy scout Danse, most of the time, my personality fits MacCready much better, I like to make fun of people too much to bring either Hancock or Preston along since they like to lecture me. Plus, I will only ever side with either the Institute and the BoS, both wouldn't be good choice to bring along for those endings anyway. 
Plus, Hancock gives you free drugs. He keeps you hook but doesn't cut you off. Best drug dealer around. 