Size
The Miniature Schnauzer is a little larger on average: about 6 kg (13 lb) as an adult, against 5 kg (11 lb) for the Miniature Pinscher.
Shedding
Their coats shed differently: the Miniature Pinscher sheds through the year, the Miniature Schnauzer holds loose hair until it is brushed or clipped out.
Exercise
Plan for a moderate amount of exercise with the Miniature Pinscher, and a modest amount with the Miniature Schnauzer.
Temperament
They differ most on affection (Miniature Pinscher: likes its own space; Miniature Schnauzer: affectionate on its own terms) and on independence (Miniature Pinscher: decides for itself; Miniature Schnauzer: cooperative, with its own ideas).
Alike
Both are rated the same on voice: very vocal.