After reading and thinking about this quite a bit, I'll have to say, I'm quite interested in this thread.
First off, I have tried Cocaine of all types. I have had Cocaine that is extremely high in purity, as well as having a high quality manufacturing process.
With that said, I do not doubt that there are active "cuts" such as Amphetamines, Methamphetamine, and other stimulants in much of the Cocaine in the US. I have noticed quite a difference in effect from batch to batch, with effects ranging from feeling extremely "geeked", paranoid, edgy, and antisocial to feeling somewhat mellow, less stimulated, and having a somewhat "fuzzy" head feeling to Cocaine that feels extremely clean, is very socially (and sexually) stimulating, does not induce high amounts of anxiety and paranoia, and best of all, every nice line is almost as good as the first (meaning you can stay high on it for a long time, whereas with some Cocaine, I end up spaced out, staring at the wall until I fall asleep later).
However, to be completely honest, I enjoy Amphetamines alot more than I enjoy Cocaine, and I am very accustomed the the effects. If there are stimulant "cuts" in Cocaine, I am willing to bet some may be more along the lines of Ephedrine and like compounds. I could be very wrong, but I know what Amphetamines feel like, and I know what they feel like mixed with Cocaine, as I sometimes use Dexedrine along with Cocaine.
Amphetamines are long lasting stimulants that have very unique psychological effects. I have never had a strong "crash" on Amphetamines, though coming down from an Amphetamine high is long and drawn out. However, when Cocaine wears off (usually after about 20-30 minutes), I am instantly hit with excessive cravings, negative thoughts, and a grinding feeling in the stomach. However, I have had type of Cocaine that do not cause this, so I know there are differences in the quality of Cocaine.
As for Amphetamines, the stages of the high are much longer than that of Cocaine. First comes the initial rush of euphoria after about 20-30 minutes after an oral dose and about 3-5 minutes after an insufflated dose. The next stage is a mix between euphoria, extreme energy, a sense of clarity, and a deep sense of psychological thought. After that comes a period of strong stimulation, though by this point, most of the euphoria has worn off. This continues to mellow out into an "awake" phase, where you just feel very, well.. awake. All of these effects occur over a period of 6-8 or more hours. I really think, with my experience, that I could differentiate between a Cocaine high and an Amphetamine high, though I have had batches of Cocaine that I suspected were cut with Amphetamines. I did notice the effects of it, though it was somewhat subtle. Normally I can tell the difference by how easy it is for me to fall asleep.
However, arguing over the effects of "coming down" is ridiculous. Any stimulant, especially a short acting stimulant such as Cocaine, is able to cause extremely unpleasant after effects. Even more so when you aren't well hydrated and use the drug without eating or taking care of your body. Though, I can say I have had batches of Cocaine that really did not cause these effects. Why? I'm not quite sure if it had to do with the actual effects of the Cocaine or my body chemistry, what I had eaten, and the fact that I may have had a good amount of sleep during those periods of use. However, I believe it is probably a combination of all.
The reason I prefer Amphetamines is because I find no real use for Cocaine other than recreation. It is purely a social stimulant and euphoriant. It fills the mind with great thoughts and ideas that never get accomplished, whereas with Amphetamines, I study, I have motivation to work, I have extreme mental clarity, and on top of all of the productive qualities, I have a high that runs so much deeper and is more thoroughly enjoyable than a high that Cocaine could ever produce.
You know, where you just feel so fucking great and everyone around you is your best friend. Talk, talk and more talk about shit that doesn't amount to crap, but is so fun to talk about anyway. No uncomfortableness at all. Sound familiar? Probably not.
That is exactly the effect that irritates me about Cocaine..
As for MDMA, I see more similarities with Amphetamines. Hell, MDMA is an Amphetamine. However, I can see what you mean, as MDMA also has a feeling of euphoria that is more basic like the feeling of Cocaine, rather than the complex inner euphoria you get from Amphetamines, though I would never find the experience of Cocaine remotely close to what you experience with MDMA.