If similarly constructed, and of similar size, there is no big difference between the two, except for the fact that in one the water reservoir is overhead of the main tank, whereas in the other it is underneath. The term "sump" simply means reservoir. Most commonly, the term sump is used for reservoirs below the main tank, and this is usually because it is much easier to place the reservoir underneath, on the floor, whereas you will need to make some sort of elevated stand for an overhead filter, and if it is large it can be quite cumbersome.
In an overhead filter, you need to pump water up to elevate it to the reservoir, and it can flow down to the main tank by gravity if desired. In an underneath sump, water can flow to the sump by gravity if desired, but must be pumped back up to the main tank.