even if you get certified through an agency, you can always go for a cross cert. the instructor i work with is a padi instructor and is getting his ssi instructor. it really doesn’t matter what agency, more what instructor. recreational certs. are pretty close through all agencys i would think. if you care about what you do you can give a new diver the education and knowledge they need. if you are already with a shop i would stick with the agency they deal with, if not padi is more reconized than any other so that would be a great place to start. good luck and keep diving.