I think one of the differences with Methamphetamine is that it can and generally is smoked which gives a much quicker onset and rush which makes it more addictive.
Dexedrine is the brand name for dextroamphetamine, the dextrorotatory stereo-isomer of amphetamine.
About 75% of Adderall is dextroamphetamine. The other 25% is composed of three other amphetamine salts.
The main reason methampetamine is smokeable is because it is sold on the street in fairly racemic crystal form (i.e. it contains both the active dextrotatory and levorotatory forms). If adderall, dexedrine and other prescription amphetamine salts were sold in the same form, free of the inert binders, they'd be smokeable too.