The way I see it they are not interchangeable.

The sander is going to take off thousands of an inch at a time and the planer will take off fractions of an inch at a time.

It is no different then using sand paper and using a plan, the sand paper will take off a little bit but with the plan it will take off a lot more.

I have a planner and of the 2, sander and planner to me the planner is the most useful for my work because I have the time to do the sanding myself but to take a 4/4 board down to 3/4 of an inch is a different thing.