the 3151's have the same power (43oz) as the 3004, so you only really get speed, resolution, and centering.

edited this in for compairison.

S3151
SPECS: Speed: 0.21 sec/60 degrees at 4.8V
Torque: 43.1 oz/in @ 4.8V
Dimensions: 1.59x0.79x1.42 in (40.6x20x36mm)
Weight: 1.48oz (42g)


S3004
SPECS: Speed: 0.23 sec/60 degrees at 4.8V
0.19 sec/60 degrees at 6.0V
Torque: 44 oz/in (3.2kg-cm) at 4.8V
57 oz/in (4.1kg-cm) at 6.0V
Length: 1.6" (41mm)
Width: 0.8" (20mm)
Height: 1.4" (36mm)
Weight: 1.3 ounce (37.2 grams)
Connector: "J" type with approx. 5" wire