The 991, 996, 998 and 999 all have the same specs when it comes to drilling. Difference is the 996, 998 and 999 have the hammer drill option which you might want to avoid just in case you accidentally have it on hammer mode, possibly damaging auger blades. The 998 and 999 can achieve slightly more power ONLY IF used with a 8 amp flex volt battery or higher. Definitely not needed with any auger type. All excellent drills.