Autosave seems like would be a better option instead of asking the user to deal with this.
I do not have any study, but the reasoning is as follows:
a) Protect user's work - as you mentioned that the form could take time, there is a risk of loosing unsaved work if something goes wrong - e.g. user clicks on something (may be a bookmark) by mistake which nvigates the user away, or user closes the window/tab by mistake etc . Autosave protects from such a loss.
b) It is not really a user preference - in other words us the user likely want it to not be saved as draft ? Most likely not. So the user is not naturally thinking about this - instead is a software concept.
c) One less thing for user to think about - Reducing user's cognitive load or following the "Don't make me think" approach .. why make the user think about this ?
Recently OSX also introduced auto-save in it's desktop app. Gmail has been doing it for a while now.
In my personal expereince, I applied it extensively in a survey software (in some ways similar scenario to what you are trying to do). It's called http://insightify.com - user feedback has been very positive.