This isn't advanced discussion. Crystal is the consistency of enantiopure dextro-methamphetamine hydrochloride in its pure form, dextro-methamphetamine freebase is an oil which one is unlikely to come across (less liable to be made in current homebake methods and not readily administrated). Racemic levo-dextro-methamphetamine hydrochloride is a crystalline powder in its purest state, but can come out looking like purple goop or 'peanut butter' and this is the 'prope-dope' made with P2P that you often hear about, having been more common before '00 in the states due to a ban of certain precursors since. The former type of meth has a smoother high, the latter has a more physical body high, racing heartbeat etc (due to higher norepinephrine transporter affinity)
In Europe I hear you have more (non-N-methylated) amphetamine HCl or sulfate than N-methylated amphetamine. I do not know what 'M' for sure is as you've referred to it; someone more knowledgeable of the drug jargon there should chime in. 'M' could for all I know be molly, or MDMA, which is an entirely other molecule of the amphetamine class with a highly different subjective profile of effects.
Otherwise "crystal" and "meth" are the same. Meth can come in the form of, what's in the U.S. known as, "crank" which is a powderized form, usually purer P2P, though 'crank' can also refer to crystal & of only the purely dextro-isomer form even.
Perhaps you should say what physical consistency your "M" takes on, what the effects are and how long it lasts.