On thei differences of resampling, please have a look here: Radar Mosaic
There is no good or bad technique, this depends on what you want to do with the result. You can calculate nearest and bilinear and compare the results.
Upsampling means bringing pixels of lower resolutions to a higher spatial resolution, e.g. 60m and 20m to 10m. Downlsampling is the opposite. So again, it depends on your output product and resolution which of both is relevant.