They're both _Really_ high up there on the "threat to life and limb" level, just in profoundly diffferent ways.
Both of these, in their modern forms, can get you addicted on your FIRST dose. From there they part company:
--The main problem with herion, in simple terms, is that it's a downer. It isn't just an opiate, but THE original opiate: you got on this to take the edge off of _everything_ to the point that your whole life is blank and rather numb. You feel no pain and couldn't care less about anything. You just sort of drift through life.
The catch with this stuff is: you build up a tolerance to it and _Quickly_, faster than even with alcohol. Nothing is ever as good as the first high on the stuff--people call this addiction "chasing the dragon" for just that reason. It takes more and more of the stuff to feel _good_ at all--and with the modern stuff that puts you at permanent risk of overdose every time. If you try to _Quit_? Beyond a certain minimal point, you get _physical withdrawal_ symptoms--you get violently ill for _weeks_ at a time. Constant anxiety, nausea and body aches, or so I've heard. And even if you don't _get_ to the point of physical dependence?
Even people who successfully quit are still at permanent risk: a part of their mind will _always_ crave the next high.
--Methamphetamines, on the other hand? They're uppers--they are what people take to _do_ two-back-to-back 16 hour shifts at a time down at the warehouse. Among other things. These drugs started life as performance enhancers commonly abused by the Nazis in the second world war.
In the Crystal form, meth is particularly obnoxious in that it is a more concentrated, intense and faster hitting high (similar to the rock form of cocaine). Suddenly you feel inhuman, angelic and insane. Who needs food, hygiene, friends, sleep, or indeed anything? One meth addict played drums until his heart stopped beating from exhaustion _days later_, another....masturbated until skin _came off_.
You go for days, living on stale air and cupcakes and loud music....and then days later, when you crash, you feel worthless, exhausted and absolutely worn out. And of course having your hygiene go to hell in the meantime doesn't help--neither do the minor burns, cuts and bruises you put on ignore while running amok. You go from feeling angelic to being the dog that gets beat.
Assuming, of course, that you get a decent purity of your drug and don't kill yourself with impurities, because that's a common thing actually. Physical dependence isn't such an issue--but emotionally, you go _quite crazy_ in withdrawal and almost always become violent.
So they're both rat poison and hell on wheels, the question then becomes: How do you want to die?