If the plan is to be a working dive bum (did it for several years, not knocking it at all), you'll most likely be in one of two positions:
1 - a dive guide that "teaches" all the time, but doesn't actually run any classes. In that case agency is irrelevant, just need to be an instructor.
2 - work at an all inclusive and you do DSDs all day everyday, in which case you probably need to be PADI.
There is also the possibility that you'll be in an instructor mill type area (Roatan, Utila, etc) where there is a lot more teaching up through DM, but those are few and far between (globally), and will probably be PADI.
Learn to service gear, compressors and/or outboard engines, it'll serve you much better than a non-PADI instructor cert ever would. Plus you can work when you are sick and can't clear your ears. A captains license also helps.