For your application I think a jaguar with an encoder reading values off of the rotating axle would be the best. The same result could be accomplished with a victor, or talon with the encoder wired to the Crio, but you would need to write the feed back loop while the jaguar has one built into it. (As far as I can remember there might be a simpler solution to an encoder feedback loop now). The motor is almost down to personal preference except that you will want it to be slow and controllable, as well as having enough force for your application. You can hardly go wrong with a bag motor on something like a 100:1 reduction. 160 rpm's and more than enough force to accomplish whatever you want as well as the same mounting holes as a cim.
Pictures are always a great addition as well, gives a better picture of what you are trying to convey
Edit: Links for bag motors
http://www.vexrobotics.com/vexpro/mo.../217-3351.html
Gearboxes
http://www.vexrobotics.com/vexpro/ge...planetary.html
Jaguars
http://www.vexrobotics.com/vexpro/mo.../217-3367.html
One type of sprocket that could work
http://www.vexrobotics.com/vexpro/sp...sprockets.html