potency, and the combinations of effects are different. I've never tried mesc but I think shrooms would be the best place to start.

shrooms will cause minor hallucinations and lots of distortions. Increased sensitivity to light, colors seem brighter. Auras around some objects, especially lights. Most people tend to laugh very hard, others gain a lasting positive outlook on life, others get terrified and revisit their most painful memories. However, In a safe setting and with a good mindset, a bad trip can be prevented or cured. Have someone you know and trust there to reassure you.

lsd is a very powerful hallucinogen. Expect to trip all day. The hallucinations will come and go periodically, and each time they'll be more powerful until you reach the 'peak,' after which you'll start to come down. Lots of patterns and fractals, lots of weird distortions akin to those optical illusions that make things move around after you look away from them. ( [url="http://break.com/index/cool_optical_illusion.html"like-a this:[/url] )

Both of these can cause emotional volatility. Choose a sitter who won't try to piss you off while you're tripping, and someone who will listen to your requests as well. You can swing between poles of emotion quickly if something good or bad happens, so it's important to be ready for that and capable of shaking a bad mood. People with bipolar should be careful, starting with low doses.

I've never tried mescaline but I think it might be somewhere in between the two as far as potency. Just talk to someone you know who does this type of thing and they'll probably be able to help you out.