Mark:
Jason makes an excellent point about small scopes sometimes requiring 2 pc. mounts. Small scopes also have less mass to control during a spring gun firing cycle.
My preference for 1 piece double ring mounts, when applicable, on "full-size" scopes is based on three things: Rigidity, alignment and friction.
Rigidity and ring alignment are pretty obvious. Friction? With the much longer base of the 1 pc. mount, there can be 4 to 6 times more contact area with a rifle's mount rail or dovetail. More contact equals more friction. Thus, with with equal or less clamping pressure (in theory) the mount would have an improved "grip" on the rail or dovetail thereby reducing the probability of "scope walk" during a springers firing cycle.
Example: Point the handle of a 10" cast iron skillet at the ceiling, then wrap just your thumb and index finger around the handle and shake hard. Now do the same thing wrapping all you fingers around the handle and shake hard.
I hope you don't have to explain a dent in floor to your wife after that first try!
