It depends on where the tank is. If visible from both ends, a corner overflow is less appealing because it takes about 1/4-1/3 of the end wall. If the tank is in a corner, definitely go for a corner overflow though.
My 75 has a corner overflow, and to be honest the obstructed view on the end doesn't bother me much at all.
Also, on a 6 foot tank, you are going to have to lean over the top to access an overflow in the middle of the tank instead of at the corner.