It's a matter of what you prefer in your players. Danny at his peak is a physical powerhouse that can bomb up and down the flank, and thus saves himself when he gets caught up high by the opposition. But a downside is that Danny has a tendency to also nullify our offensive player out on the left since he doesn't leave them enough space to operate.
Ben is not as pacy (but he has improved in that along with his tackling and overall physical play. Was a beast this season when required), but works around that by being a better reader of the game. He hugs the line and moves to where he has to be before he has to be there. Starts runs early but generally tends to not move as far forward as Danny, giving our attacking player on the left more space (It is not a stretch that Son tends to have better games when playing in front of Ben than Danny). And when Ben moves forward he tends to cut inside and breaking through defenses.
Danny is more a wingback and Ben is more a all around fullback.