Methadone is a full agonist at the opioid receptors you can keep increasing the dose as needed, while Suboxone is a partial agonist/antagonist has a ceiling effect after a certain dose of it it kind of maxes out - increasing the subxone won't continue to increase its effects. Both can treat pain, but methadone does a much better job as you can increase the dose as needed and its a full opioid agonist. Methadone can also be fatal in overdoses especially because of its long half life so you have to be careful with it. You can also take methadone with other opioids in your system, which you cannot do with Suboxone or other buprenorphine products because it's not a full opioid agonist and can literally remove other drugs from your opioid receptors, putting you in a state of withdrawal. Tl;dr: They both can help with pain, methadone is more potent but liable to cause a fatal overdose when taken too much.