Converting my comment into an answer:
pgf v3.0, the
datavis library is released to CTAN (no longer CVS-only). So the objective answer is "use whichever you prefer". You will likely find proponents on both sides of the fence. But there is a greater user/community support base for
pgfplots because, as you note, it is more mature at this point.
I personally prefer
datavis currently feels too verbose/pedantic for my taste. I'd rather have the
pgfplots approach of "there are defaults here; customize them if/when needed". But again, this part is opinion-based. My opinion may change over time, and others have different opinions of course.
With a bit of time invested in learning, you can produce beautiful plots with either system. Happy TeXing/plotting!