If you were asked to pick one, really either one is extremely helpful for getting into graduate school. Great letters will speak to your potential as a researcher, and great research shows it fairly concretely - but as has been noted, it may be hard to recognize research quality outside of a particular subfield. In my field however (public health) there are some broad strokes measures of quality that someone can pick out. And if we're talking about publications, that's rare enough in my field to be an unusual entry on someone's CV.
Moreover, I'd suggest that this dichotomy is something that likely won't exist, especially if you do great research. A good letter discussing that research, cementing its contribution in the admission committee's mind and setting it up as "a preview of things to come" should follow pretty naturally.