The difference is in the size of the AHPCO Cell. The larger the cell the more powerful the performance of the unit. The 3000G3 would be used in a medium sized home under normal situations. The 3000Xtreme is recommended when there is higher contaminant levels in the same space. That could be severe mold contamination, odds from numerous pets, chemical smells, high VOC levels from paints, glues, new furniture, pressed woods. etc.