Slice Assembly is specifically developed for Sentinel-1 products and can be applied before the geocoding.
SAR Mosaic is a way to merge different coverages in both range and azimuth direction. I would apply it only to stripmap images which are already geocoded by Range Doppler Terrain Correction. You can define a fuzzy area (feathering) to blend from one image into the other if they have overlapping parts.
Some remarks on this: GRD BORDER NOISE removal over ocean areas - #2 by lveci