hello. wellbutrin XL is extended release and SR is sustained release. dont know much of what the difference is, (i've been a pharmacy tech for five years), i think it has to do with what intervals the medication is released. (ie, i think one is released slowly over time, and the other is released repeatedly at other intervals). there is currently no AB rated generic for wellbutrin XL 150 mg, only the 300 mg XL and all of the SRs (which is bupropion er.) it is also available under BUDEPRION er; though budeprion is technically the AB rated generic for ZYBAN, a smoking deterrent.) but, technically the active ingredient/chemical in both is Buproprion.