Base vs Mid ?
The mid model contains an extra 3" and 4" bolt. The extra 4" bolt allows a 1" wider stack for 10~15 more pounds on the high end. On the low end, the extra 3" bolts lets you use the more versatile two-bolt setup down to a stack width of 6" (3"+3") while the base model has a minimum of 7" (3"+4").

