Snapchat to me is sharing your shaky drunken escapades at 3AM with your friends to let them know you made it home and survived the night. Instagram seems more like an endless observation of copy-and-paste, superficial things and people and places. It's evolved more into a (usually inaccurate) portrayal of status than anything else.
snapchat is fleeting. the quality can be lower. no one is gonna care. and the fact you have to take the picture then is more exciting than posting any old pic you found online. "this is what I'm doing"
I can snap you my beer, I'll insta the edited GoPro footage of me cliff jumping