A servo with the electronics removed is just a DC motor with a gearbox. You need an H-bridge circuit to drive it from your microcontroller.
This is the case for the HS-5475HB servo from Pololu. This servo has karbonite gears and more torque and a little more speed (54 RPM compared to the other servo's 52 RPM at 6V). You still need to clip the limitation nub from the spline gear. I am not sure if I shoud recommend this servo or not, since it does not have enough speed for a fast robot but it has a little more strenght than a regular servo. But you can use the empty space inside to add an encoder.
The HSR-1425 CR servo has the electronics inside, it is allready modified for continuous rotation, the electronics are modified to acheave fully proportional speed (verry good!). This is perfect for a small robot, based on the $50 robot design and code. You don't need to do any modifications on this one.