LSD does not stay in your body forever; this is flat-out false. Nor does it damage your chromosones. There is no measurable long-term physical consequence from one hit of LSD.
It could have long-term psychological effects, but so could any intense life experience. Sometimes it can trigger pre-existing but latent issues.
Mainlining any drug is more dangerous, since any impurities are also taken more directly. But since this is a hypothetical, we can assume both drugs are known pure.
Heroin is more addictive physically and mentally, but hardly to the point that one dose hooks you for life. In fact cigarettes are far more addictive statistically.
So, I'd probably say heroin because that's the one I haven't tried yet :-)
Like every other drug question, your research should start with
http://www.erowid.org .
To repeat, the LSD-in-the-spine thing is pure myth. Not that I necessarily endorse it, but we should get the facts right before forming opinions.