Unsuccessful research project paper as writing sample?

[pebs]

I'm applying to a Ph.D. program directly out of undergrad, and I have strongly-written paper for an undergrad research project that is a shining example of my interests.  It would be a no-brainer to use as my writing sample, if I hadn't had to truncate the project due to problems with the data that only made themselves known late in the game when running the numbers produced no valid results.  My research supervisor still had me complete a paper for course credit; this paper has many elements of a typical research paper (presentation of hypothesis, background, lit review, suggestions for further study, etc), but the bulk of it is an examination of the data, the methods, and why the project went wrong.  (To this end, it is more conversational in tone than it would otherwise be.)

I've had one professor say he doesn't think the project's outcome would reflect poorly in the context of a writing sample, and another tell me the opposite.  I trust them both, and would err on the side of not submitting it, but my other writing samples are just not anywhere near as good an example of my ability to do research.  One is a seven-page paper on a topic well within the scope of my degree field, but nowhere near my research interests, and the other is a 12-page co-authored lit review for which I did the bulk of the work.  Each is well-written.  Barring using the failed project paper, I would submit them both (with explanations for the co-authored paper), but I feel like I'm hurting myself by not showing my work in my area of interest.

Any suggestions?  Should I stick with the other two, and maybe excerpt the troublesome paper?