I would go for the JR DS821 , cost for each is less than 30 dollars.

They'r digital and here you can compare both.

Futaba 3151 [:'(]

SPECS: Speed: 0.21 sec/60° @ 4.8V
0.17 sec/60° @ 6.0V
Torque: 43 oz-in @ 4.8V
54 oz-in @ 6.0V
Dimensions: 1.59 x 0.79 x 1.42 in (40.6 x 20 x 36mm)
Weight: 1.5oz (42g)

JR DS 821 [>:]

SPECS: Speed: .19 sec/60° @ 4.8v,
.15 sec/60° @ 6v
Torque: 72 oz/in @ 4.8v
88 oz/in @6v
Dimensions (WxLxH): 0.74"x1.50"x1.47"
Weight: 1.5 oz (42g)