Adderall contains both isomers of amphetamine, dextro and levo. Dextroamphetamine is about twice as potent as levoamphetamine. Dexedrine contains only the dextro isomer or amphetamine which is why those who have tried it will probably tell you it is better.

Don't do meth thinking it wont show up on drug tests, it is included in the standard drug test which tests for amphetamines (including meth).
http://www.erowid.org/chemicals/meth/meth_testing.shtml