This is really quite debatable. Sure, if we look at the pharmacodynamics of both drugs, we can certainly see a far greater degree of toxicity associated with meth. Besides being a proven neurotoxin, meth can have horrific effects on the cardiovascular system, most notably sustained high blood pressure and tachycardia.
But when people talk about how "harmful" a drug is, I believe many harmful traits of drugs are ignored, such as they're impact on personality, morality, finances, etc. Heroin, in my opinion, is far and away a more addictive substance, and I have seen people's lives reduced to shambles as they succumb to the addiction. Sure, meth as extremely addictive itself, but heroin addiction redefined what I called an "addiction." Theoretically, responsible use of heroin would be more or less benign, but come on people, how many people use heroin "responsibly?" I'm not saying there aren't people who can do it, but a good portion of people who use heroin regularly become dependent. Put it this way--I've met more people who dabble in meth but avoid addiction than those who dabble in heroin.
In my experience, most people who use heroin like to bang it, or at least end up IVing at some point. While a lot of people bang meth, I've met a lot more meth smokers than heroin smokers. Not saying that there aren't circles where that's the norm--around here we don't see much tar, which I understand is better suited for this purpose--but from what I've seen, people usually tend to shoot smack. And of course, IVing has a whole host of complications. Sure, if people were all completely sterile, rotated injection sites, had flawless technique and micron filtered it might not be a problem. But I've seen way too many addicts shoot with dirty needles and without a cotton to know this is not the norm.
Meth is also considered dangerous because of the potential for sudden death resulting from a stroke, cardiac arrest, etc. But I wonder if, statistically speaking, heroin users run a similar risk of death from overdose.
I will say one thing though: I've met a lot more old heroin addicts than old meth addicts. It seems like it is possible to user heroin for years and years and not suffer any noticeable long term effects, but meth, that shit'll put you in the ground.
Personally, I prefer heroin to meth any day. I'm just not going to delude myself or others into thinking there aren't significant risks associated with it.