^^ Regarding K'd Out's thoughts on desoxyn.
I wonder how many others agree w/ you that desoxyn is 'smooth and doesn't have that up-down, up-down feeling', as I was prescribed it for a bit (less than a half year ago, took it for a few months before switching back to Adderall, currently not prescribed ANY stim) and noticed a severe come down and thought it to be extraordinarily harsh. It felt 'dirty' to me. I had wicked headaches, didn't feel that same rush that adderall gave me. I could FEEL its potency in a way that is difficult to describe, but it did very little FOR me.
Back to original question (and such) : XR is the only form (vs IR) that comes in capsule form/bead form -- at least in my experience (anyone disagree/know that this is incorrect? I'm not saying that IR capsules do not exist, just that I have never seen/heard of them: thus, please set me/others straight if need be!) ** Also, as others have stated, you really just have to find out what works best for you. One really cannot say that a particular medication (in this case, for ADHD/ADD) is 'the best' as it is extremely subjective. Reasoning why there is a salt combo? - efficacy, as stated above. These salts in combination create a substance that is most useful for the purpose at hand. I definitely don't believe wikipedia is 100% accurate, but do think it has some great info and is easy (or, easier) to read in comparison to various academic documents/research papers -- for the average person

but check it out! Read through it, it is actually quite interesting and informative (+ answers your question/s to a decent degree)
http://en.wikipedia.org/wiki/Adderall