I have both.
The Grand Slams ride BDL's in .223 and .243 and the 6 X 42 is on a BDL 7 Mag. The Leupold has more eye relief and keep the scope from biting my fore head so much. The Grand Slams are Clear but do not have sufficient eye relief for me in the hard kicking Magnums. I am a self professed "Stock Creeper" and require maxium eye relief to keep from bleeding between the eyes when I shoot. However, the Grand Slam are a lot of scope for the $$'s.