shooting heroin isn't any more or less addicting than snorting it. it's still the same drug, you just get more out of the experience because when you IV certain opiates like morphine, heroin, fentanyl, etc. you experience a rush. when you snort heroin, the euphoria comes on gradually over a period of 1-2 hours. when you inject, the euphoria hits you all at once so that peek plasma levels occur the instant you inject it. because of this, the effects are much more dramatic. when i inject, i usually feel all the lights around me dim suddenly as a result of my pupils constricting and during that same moment sounds will become muffled in a weird sort of way. now, you experience these same effects when you snort the h, but it won't be as noticeable because the effects come on more gradually. it doesn't really have much to do with instant gratification since snorting heroin and injecting it are pretty much both forms of instant gratification. the experience is just completely different because the h hits you like a ton of bricks at such a high concentration instead of slowly being absorbed through your mucus membraine.
i choose to inject heroin because the feeling is incomparable to insufflation. i don't do heroin that often so i try to get the most out of it this way. there are tons of opiates out there like buprenorphine, codeine, hydrocodone, ms-contin, etc. which i don't inject because you can't get a rush from them, or they're much harder to prepare for injection, but when i do easily injectible opiates i always IV it. those who think they won't get addicted to heroin if they just snort it are kidding themselves. i know tons of people who have never injected anything in their life who are hooked on all sorts of drugs including opiates. in fact, one of the people i know who's the most addicted to opiates has never touched a needle in his life. if you can exercise moderation in your recreational drug use, then your method of ingestion doesn't really matter. if you can't exercise moderation or you have an addictive personality, then you're fucked either way.