Size
The Miniature American Shepherd is larger on average: about 14 kg (30 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 Miniature American Shepherd needs brushing a few times a week.
Temperament
They differ most on affection (Miniature American Shepherd: affectionate on its own terms; Shetland Sheepdog: a lap or leaning dog) and on handler demand (Miniature American Shepherd: needs steady training; Shetland Sheepdog: suits a first-time owner).
Alike
Both are rated the same on independence: keen to work with you.