They really are just very different trips. As others have said, ounce for ounce (and lord help you if you dose an ounce) LSD is the most psychoactive drug in existence. Threshold effects are measured in micrograms. However, that doesn't really mean much from a practical standpoint.
For me, acid is a much clearer, more intelligent trip. You perceive the world in entirely different ways, but maintain the capacity to analyze what you are perceiving, even if you cannot properly express yourself. There's a definite connectedness and a sense of "meta-" if that makes any sense. Bad trips are rare, and almost always a product of you being in a mental state that does not jive with your environment. If you are mentally and emotionally comfortable, you will have a good trip, I guarantee it.
Mushrooms, on the other hand, are confusing as fuck. You will walk around in a daze for your entire trip. You will be entranced by shiny things and colors will look fucking awesome. Be careful though, it's easy for a mushroom trip to go south and you will definitely have less control than with acid.