Hello,
Both will work but you’ll have your box (sphere) around you allways and will need to deal with even when you don’t have enemies (by cast to for example). On the other hand a detection box around your ai character will be usable for more than only the enemy ( change movement direction / destination if box overlap electric wall or pit for example).
A good example, easy to implement is : https://forums.unrealengine.com/showthread.php?25415-Basic-Enemy-AI-(BluePrints-Only)-(Project-Link)&p=108544&viewfull=1#post108544