Both are good, but the 625 will have more torque and same speed.

JR Z590:
Torque: 85 oz/in (@6.0V)
Speed: .15 sec/60 deg (@6.0V)

Hitec HS-625MG:
Torque: 94 oz/in (@6.0V)
Speed: .15 sec/60 deg (@6.0V)

Not sure how much the Z590 sells for but most places sell the 625MG for $35-40.