undergraduate research experience should go for 'depth' or 'width' in terms of graduate application?
As long as you have something, than that is good enough.  You will be working on a new problem during your Ph.D. and it will be very different in the amount of time spent (full time, versus an "extracurricular").  Your grades, test scores, and letters are more important.  Nobody is expecting you to be an independent researcher at this point.  They are much more interested in scientific potential than results when rating undergrads. 

I would do what you like the most. (Life is short and then...)  But if you force me to vote only based on your application, then spending more time on the initial problem and getting a publication (or more pubs) is the higher return on your time.