I'm a massive fan of mescaline - I consider it the King of Psychedelics. A single high dose of this stuff changed my life from being on a collision course with tragedy to a life on a significant upwards trajectory.
Cactus vs HCL is a good question. The first time I did it, I had peruvian torch. I ended up sticking it all in a blender with a smoothie and other bits (I intended to do 50g, but it was more like 80g in truth), and after necking two pints of the most god awful thing I've ever put in my mouth, I had probably the most introspective, profound, and transformative trip I'm ever likely to have. I've had some awesome trips, but this was my first ++++, and even those that have come since seem mediocre in what this trip did for me.
I've since attempted cactus a couple of times, but not reached that peak I had on that trip. Nowdays, I'm a bigger fan of HCL as its simply easier on the stomach, but again, even at large doses, I've been unable to replicate that first time. Most I've done on HCL is 800mg, and whilst it was quite the experience, it was hampered by an LSD trip from the week before. I'm planning something of equal strength but no distractions at the end of the summer.
The MDMA bit is right, but its more like candiflipping MDMA with a psych. You get an immense body high, and will feel entirely at ease to discover strengths within you that you never thought you had. Growing the cactus is a bit of a commitment - it takes at least five years! I'm planning to get some for my own growing purposes once I next move.
Oh, and as for the nausea, take enough mescaline and you'll be perfectly happy with your sick gut

Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside in a cloud of smoke, thoroughly used up, totally worn out, and loudly proclaiming "Wow! What a Ride!”
― Hunter S. Thompson