Even the worst trip experience is unlikely to be traumatic in and of itself unless it triggers some latent condition or bring up some previously repressed trauma (e.g., if you were abused as a child, repressed the memories, then had them resurface during a bad trip). No matter how horrible the experience itself is, within days or weeks when you have had time to think back on the experience, you will understand that the negative experience was the result of being under the influence of psychedelic drugs. A memory of being raped is quite different; it's a memory of a violent sexual attack by another conscious being. It's not just the emotional and physical suffering of a negative event that makes it traumatic, but the way we make sense of that event, the way it relates to our sense of self, our cultural and social mores and values, etc. Most traumatic experiences involve the death of a loved one or an act of violence carried out consciously.
Ultimately, attempting to quantify things like this in abstract terms is probably doomed to fail. Trauma is a condition experienced by a conscious mind, not a property of events or things. The context a negative event occurs in is more important than the physical nature of the event itself. Since rape necessarily involves a violent, disturbing context whereas bad trips do not, it seems reasonable to generalise that rape is more traumatic in general.
On a personal level, I would absolutely pick a high dose of psychedelics with the knowledge that I would have a terrifying bad trip over being raped, and I imagine most others would as well. That may not exactly be fair since much of the terror of a bad trip is that you don't see it coming, but the same could certainly be said of rape. This thought experiment is becoming morbidly ridiculous, so I'll end it here.
