This isn't really a matter of harm reduction but rather one of basic curiosity, so I sadly gotta bring you down. What it all boils down to is personal preference. Any junkie will love all three of these drugs with an essentially equal vigor. It's like having a certain wine to go with your pork or whatever the fuck. It just works and it's your favorite for reasons that are essentially due to a personal decision, the genesis of which is not known even to the user themselves.
Ask me for instance. I'm a junkie. I've done my time. Heroin was my go-to of course, but loved these other three just as much. I know this is old hat, but Heroin is only demonized because it was truly the first serious Opioid epidemic of the 20th century. It had not been seen in such a form prior and the name "Heroin" would forever be associated with bedlam, meanwhile even it's fraternal twin, Morphine is just fine. If we were doing all of these drugs in their pure forms Heroin would be just another addition and let's get real, if you get high, you're not complainin right?