I think there are positives and negatives to all options.
1. External overflow
Positive = less room taken from internal tank space = more room for scape or free swimming space
Negative = more external space taken = home placement challenges & cant be placed flush against wall without leaving a gap/space
2. Internal corner overflow
Positive = less obstructive placement than having it in center which permits more scapeing freedom and can be placed flush against wall
Negative = takes up more usable tank space and unless you place tank in a corner you now have a visible overflow from outside of tank unless covered / painted
3. Internal center overflow
Positive = Freedom to place tank flush against wall and still be able to be visually appealing from other 3 sides without covering or painting to cover exterior eye sore
Negative = takes up space in the center which can cause scaping challenges
I currently have a cube with an exterior overflow and I am currently in the planning stages of a new larger cube which will be internal overflow as I do not like having an external overflow due to placement limitations.
Good luck with whatever you decide