What's the difference between?

The RocketPropulsion object is used to apply force to a Part or other object in a similiar manner as a rocket (ie. towards a given part, with given thrust and torque).

a body position is a way to position something using ROBLOX's physics. The object will try to zoom to the point identified by the position, and will try using the amount of force defined. Now this can be helpful. What if you define that there is a lot of force on the Y axis (Up/Down), but not on a lot on the X/Z axis (Left/Right and Forward/Back). In this case, you created a floating brick, that can move anywhere. This can be quite helpful in creating hover crafts or ships. Another useful thing, is to remember that it will try to get to the position, but it won't necessarily get there. So if a brick that is anchored is in the way, that part probably won't get to the objective.