First, you should read this. It's a schwak of information about motors and their components.
Motor Explanation on XMW by CreativeSN.
Second, 130 motors (the stock motor in your Xmod is a 130 size motor) have no gear reduction units on them like the servo's and the Tamiya Gear Box's have. These make the torque many times more prominent than that of a 130 size motor.
Here's a visual representation of both of them being used in an Xmod:
Servo motor:
Tutorial on how to do the servo mod with an HS-55 servo found here.
Photo by Lancer_Man.
Tamiya Gear Box:
There is no known tutorial on how to do this yet. I'm sure Texan would be able to help you if you choose to go this route.
Photo by Texan_Idiot25.
As you can see the HS-55 conversion is much smaller but probably has less torque because there are less gears to do the reduction. Also, there is minimal modification needed to be done on the Tamiya Gear Box where as in the servo modification you have to take the whole servo apart to make it spin 360*. However, in the Tamiya Gear Box you must hack away at more of the motor cradle than you would have to hack away at for the servo.
It's all personal prefference. One's smaller, one's larger, one takes more work, one doesn't take as much work..