If you are tight on time, say 2 days, I would recommend spending a full day in Zion and travel to Bryce the next day and have a half day or so there [1 night]. Both parks are spectacular, but Bryce can be enjoyed quicker from the viewpoints along the edge of the rim that overlook the stunning Red rock spire formations. Zion has many easy and short walks available through the canyon and takes a little longer to explore and enjoy IMO.