Size
The English Cocker Spaniel is a little larger on average: about 14 kg (30 lb) as an adult, against 11 kg (25 lb) for the American Cocker Spaniel.
Shedding
Their coats shed differently: the American Cocker Spaniel moults most as the seasons change, the English Cocker Spaniel sheds through the year.
Temperament
They differ most on handler demand (American Cocker Spaniel: suits a first-time owner; English Cocker Spaniel: needs steady training) and on prey drive (American Cocker Spaniel: usually fine with small pets; English Cocker Spaniel: chases; needs managing).
Alike
Both are rated the same on affection: a lap or leaning dog.