LSD and Psilocybin Mushrooms AKA Shrooms are the two most widely used psychedelic drugs, at least in the western world that is. Users range from curious teens to older psychonauts, and the portrayal of these substances in the media may lead one to believe that they're interchangeable as far as their effects on human consciousness go. This is simply not true, and this widespread belief could lead to some inexperienced users getting themselves into a nightmare of a trip, especially if it's their first time.
Lysergic acid diethylamide (LSD)
LSD, commonly referred to as Acid, is a synthetic compound unlike Psilocybin. It was first created by Albert Hofmann in Switzerland in 1938 and is actually one of the more difficult substances to synthesize, which is part of the reason why you don't often hear about "LSD Labs" being raided by the law. As far as it's objective effects go, LSD will last quite a bit longer than mushrooms will, with most trips lasting 9 - 12 hours depending on how large of a dose is taken. However, one should expect the peak to only be 3 - 5 hours long, again depending on how much is ingested.
Subjectively, I can only offer up my own experience, although some of it is supported by the general consensus of those who experiment with psychedelics. LSD is a lot less "intoxicating" than shrooms are, although I say that very lightly. What I mean is that especially at moderate doses, Acid will alter your thought patterns and headspace substantially less than mushrooms will. However, it is still a lot more intoxicating than something like cannabis is. You should never expect to successfully hide the fact that you're tripping on LSD from someone who believes you to be sober. It's not only obvious, it's a recipe for a terrible trip. Aside from your behavior and altered state of mind, the most obvious physical giveaway that someone is on a psychedelic drug is the fact that most of the time their eyes will dilate heavily, (see my profile picture for reference). A lot of people state that Acid is a much more visual experience than mushrooms are, but I've not found that to be the case. I personally get a lot less visually speaking out of LSD than most people do, which is a testament to how differently substances affects people, especially psychedelics. And due to it causing less of an alteration in headspace than mushrooms, it is much more suited to be used in a more recreational sense than the latter substance is. For example, I would never take shrooms in a very public setting like a concert, but taking acid on the Vegas strip is on my bucket list.
Psilocybin on the other hand is a naturally occurring compound found in countless species of mushrooms across the globe, with the most potent being those of the genus Psilocybe. These "Magic Mushrooms" are known to have been used by humans for thousands of years, and only in modern times have they been widely seen as something negative and prohibited by the law. We have the American War on Drugs to thank for that, Thanks Obama! Unlike LSD, it's not definitively known as to whether or not Psilocybin itself has any psychoactive effects. This is due to the fact that it's a prodrug, which essentially means that the human body converts it to another substance upon ingestion. In this case being Psilocin, which is where the known desirable effects of the drug come from. In fact, there even exists other prodrugs for Psilocin. The most notable one being 4-Acetoxy-N,N-dimethyltryptamine, which is more widely known as 4-AcO-DMT, and produces virtually indistinguishable effects from mushrooms.
The length of a mushroom trip is much more dose dependent than LSD as larger amounts will require a longer amount of time to digest, but generally speaking if a standard dosage is ingested you can expect the experience to last around 4 - 6 hours, with the peak lasting only about 2. This is a much shorter trip than that of acid, however it is an endeavor in which a great deal more amount of thought should be put into set and setting as you can generally expect a more intense time. Shrooms provide a lot more of a headspace than LSD, and it's far easier to have a bad time on them. While I've never had an acid trip which has considerably altered my viewpoint of the world, I've definitely had a shroom trip that has. Which is a testament to how powerful of a substance Psilocin is. This is just my own personal experience of course, however most experienced users would tend to agree with me. Visually, I've always had far more intense trips off of mushrooms than I have from LSD, but the visuals are a bit more "foggy" as the trip lacks the clearheaded feeling that characterizes acid trips. I'm speaking of moderately dosed experiences of course, as if you take enough LSD it'll definitely have you floored
For The Inexperienced
For any first timers looking to dabble into psychedelics, my own personal recommendation would be to try LSD first. This is assuming that you only have access to these two substances as there are milder options out there, they're just harder to come by. There's a commonly shared quote which states that acid is like driving a rocket ship through space while mushrooms are more akin to being strapped onto the back of a rocket as it shoots you wherever it pleases (Or something to that effect, I've seen it stated differently depending on the source). I only recommend LSD if you know 100% for sure that what you're taking is actual acid though, as in many cases tabs procured from the streets are dosed with other substances. While LSD is completely physically safe for healthy individuals to consume, a lot of these other drugs are not. So if you do not have access to a test kit then stick with shrooms.
Don't by any means take this as me saying that you don't need to put thought into your set and setting when taking LSD, because you absolutely do. It's just a tad bit more forgiving of an experience than mushrooms are, however you can absolutely still have a nightmare of a time on it. In fact, a friend of mine recently had a psychotic break due to carelessly consuming acid, so always treat these things with respect. Be safe and have fun!