It should depend on the school. For example, UCSF looks over both applications at the same time (previous app & current app) and they suggest (almost demand) that you rewrite your essays, especially the supplemental app because they will be looking at the apps almost side-by-side. At least that was the impression I got. Also, you need at least one new letter of rec, because they review the old letters as well as the new ones. Therefore, I'd feel like re-applicants have a stronger chance of getting in [at UCSF] since they basically get twice as much information to convey to the committee.