Using five years of observations from the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3), we assess the changing sensitivity rate of the two WFC3/UVIS charge-coupled devices (CCDs) and evaluate the photometric repeatability of the spatial scan observing mode in comparison to the standard staring mode. We perform aperture photometry on vertical, linear spatial scans of two white dwarf standard stars (GD153 and GRW+70D5824) taken on corner subarrays of the WFC3/UVIS detector, and compute the rate of photometric sensitivity decline from 2017 to 2021. To gauge the relative accuracy of scans, we compare sensitivity losses for staring mode observations over the same 5-year time scale and those acquired over longer time scales. After removing the time-dependence of the relative photometry, dispersion of the residuals is used as a proxy to measure repeatability of observing modes, and thus assess precision. We establish that spatial scans are more precise than staring mode observations. Scans with UVIS 1 show 2.4 times less residual noise than their staring mode counterparts; for UVIS 2, residual noise for scans is 2.5 times less than residual noise for staring mode. For scans, sensitivity losses are relatively flat independent of wavelength on both UVIS CCDs, with no evidence of contamination. UVIS 2 appears to have slightly higher losses (-0.17 +/- 0.01 %/yr) compared to UVIS 1 (-0.12 +/- 0.01 %/yr). When measured over the same time period, spatial scans and staring mode observations yield filter-dependent loss rates that agree well with each other in most filters within computed uncertainties.