disclaimer: starting shooting is never really a good thing, just giving my experience.
For me shooting heroin really didn't hurt me any worse then I would have sniffing it, I love shooting my dope.. but probally like the act of shooting in itself as much as the actual high. For me I use heroin for the ease of mind and sniffing or shooting both acclompish that very well, I was addicted for awhile pretty bad but I'm sure it would have happened no matter what my roa was. I recently starting using dope again mostly shooting but just twice or so a week now.
On the other hand shooting cocaine and crack was one of the worse things I ever done instantly fell inlove with the high and never been big with sniffing coke, and shooting heroin didn't have anywhere near the same effect.
But everyone is different probally best to just let sleeping dogs lie, but I just personally never found shooting heroin that much worse then sniffing.