Willy-
  I sold that Amulet Glow to PMJ awhile ago.  I also own a Slate Blue.  The Glow for me was long, but actually had a snap on dry backends.  The Slate Blue starts much earlier and is more of an arcing backend.  I had success using both on drier lanes.  For fried lanes, the Amulet is better, because even with the pearl urethane cover of the SBG, it will read the dry VERY early.  The hit on both are very good, with the SBG getting a slight edge.  I personally don't think of either one as ideal spare balls.  If I had to pick one though, I would use the Amulet.  The SBG makes for a good drier lane control ball, but I have found that it needs a bit of oil to get it down the lane.  I hope this is what you were looking for.

Don