Size
The Pembroke Welsh Corgi is a little larger on average: about 11 kg (23 lb) as an adult, against 8.5 kg (19 lb) for the Shetland Sheepdog.
Grooming
The Shetland Sheepdog needs more grooming, daily brushing, where the Pembroke Welsh Corgi needs brushing a few times a week.
Temperament
They differ most on sensitivity (Pembroke Welsh Corgi: hardy, takes a busy home in its stride; Shetland Sheepdog: needs a calm, predictable home) and on affection (Pembroke Welsh Corgi: affectionate on its own terms; Shetland Sheepdog: a lap or leaning dog).
Alike
Both are rated the same on independence: keen to work with you.