Size
The Sealyham Terrier is a little larger on average: about 10 kg (22 lb) as an adult, against 8 kg (18 lb) for the Cesky Terrier.
Shedding
Their coats shed differently: the Cesky Terrier sheds through the year, the Sealyham Terrier holds loose hair until it is brushed or clipped out.
Grooming
The Cesky Terrier needs more grooming, daily brushing, where the Sealyham Terrier needs a brush once a week.
Temperament
They differ most on independence (Cesky Terrier: cooperative, with its own ideas; Sealyham Terrier: decides for itself) and on handler demand (Cesky Terrier: suits a first-time owner; Sealyham Terrier: needs steady training).