Continuous rotation 'servo' vs DC motor
I can't control the speed of the servo, can I?


Yes, you can, but not to a very good degree of control


  Also, it must have something akin to an h-bridge inside the motor, which surely ups the cost, but the motor will always be geared, which is a plus.


Continuous rotation servo:


Gearing
Built in motor driver
Single pin control (forward, reverse, speed)
Easy to mount the servo, and to attach movements to the horn
Hard to 'center' due to temperature drift - ie, without feedback you can't ever fully stop a servo, and even with feedback it continues to hunt for 0, and induces vibration into the mechanism


Motor:


Cheaper (even with gearing, especially in quantity)
Greater degree of control with a good motor driver
Doesn't require a PWM signal
Smaller
Lower power consumption for most applications