The whole point of Snapchat is that it's not about sitting down, taking a high quality photo, editing it, etc. It's a messaging app meant for quick replies. You send someone something and they reply back. That's fundamentally different from Instagram's model of user interaction. It's like comparing time spent in iMessage to time spent on YouTube: it doesn't make sense.
When it comes to viewing (which is where ad revenue is generated), both apps have similar customer behaviour.