shape keys/ipo drivers

these appear to do the same thing. Is one a preferred way of doing things over the other during animation?

They are not the same thing. Shape Keys let you assign a target shapes to a mesh so you can “morph” between them. Setting keys for these creates IPO curves for each shape, but does not create drivers. You can manually edit the IPO curves to tweak the shapes’ influences.

You create IPO drivers so that animating Size, Rot, or Loc on one object controls an IPO curve (of any type) on another object. So instead of the driven object’s IPO changing with time, it changes with respect to the first object’s movements. They can be used to drive Shape Keys, but can also drive changes of colors, movements, wind forces, or anything that has an IPO.

Wrt shape keys, IPO drivers are good for complex setups like facial animation. You don’t need them for simple shape changes like morphing an object once.