Hmm. All other things being equal, I suppose I'd see them as equally far out into fuzzy boundary land, since they both lack a central characteristic of fanfiction--in one case the derivativeness, in the other the fan community and mode of production/distribution and whatnot. That said, things are rarely ever equal and I'd expect there to be mitigating factors: since the original slash is coming out of the community it is more likely to utilize fannish tropes, aesthetics, genres (not only het, m/m, femslash, but PWP's and WIP's too), etc. Then again, some things are universal--it might be more likely to see a Mary Sue in drawerfic! (Insofar as writing from the id is one of the axes, that'd be generally more prevalent in drawerfic as well, probably.)