GRLevel3 is a Level 3 product viewer, designed to view live radar data, particularly when field spotting or chasing via laptop, as Level 3 data, while lower resolution, is much smaller to download. It is also a bit more difficult to view Level 3 archive data (see the GRLevel3 archive question for a how-to).
GR2Analyst, while it can view live data, was primarily designed as an archive viewer and features analysis tools, including volume rendering.