Either would be fine. 6v will give you a bit more torque from your servos, so I would go for that one personally.
As long as it’s between 5 and 6v for the servos, and enough current supply, you should be fine. You may need to supply the logic with a separate 9v battery or something, as those power addapters tend to drop their voltage when high current is drawn by the servos, which resets the SSC32.
You’re applying the wrong logic to the problem. You can use a 4.8 to 6.0vdc supply capable of providing 2.5 amps or more… More current capability will only help the situation.