90% of the models I'm currently flying use 4.8v packs. But, most servos will give a little bit more torque and speed with the extra cell.

Here's the difference it can make with the S3151s. The truth is I would probably never notice it in the air, but it does give you a little more for your money.

Speed: 0.21 sec/60° @ 4.8V
0.17 sec/60° @ 6.0V

Torque: 43 oz-in @ 4.8V
54 oz-in @ 6.0V

Same servo, different specs simply by adding one cell.