I will get fairly detailed for you here, as it sounds like this is your first year in FTC. power/torque/speed/battery-system/positionability/COST/dependability - are all factors that will guide your choices.
NXT motors are more along the lines of a continuous rotation servo for FTC purposes. They are powered by the 8v (or 9v) volt battery system on the NXT. They are large and can be harder to mount than the boxy tetrix servos - but, you have 3 already and don't have to order any additional 'continuous servos'. You can attempt to control these motors using degrees - test thoroughly and very simply to see if the results meet your expectations.
The Tetrix servos provided are 180 degree positional (not continuous) - and quite accurate and dependable (over a limited 180 degree range of movement).
The DC motors are much more powerful, and powered by the 12-volt battery system (along with any of the servos you use with the Servo Controller, and the Samantha Module). Clearly, these are required for your DRIVE motors. You can drive 4 motors from one Motor Controller for DRIVE TRAIN (hard wire the pairs - Left and Right - to the same motor ports on the DC Motor Controller). You have to order a second Motor Controller to begin operating additional DC Motors on your robot. When programming - do not attempt to control motors that are on different Motor Controllers by the same MotorMove block - each Motor Controller should have it's own MotorMove block. You can install the 'encoders' on some of these motors to allow feedback (ie. degrees of raotation). These are very expensive - as I recall, you are given 2. We have yet to use them, and simply focus on power/time to control our movement (move while pressed for Driver Control period). Some teams say they wouldn't do anything without 'encoders' - personal preference.
Here is a sample spec sheet on the Tetrix motor: http://www.patronumbots.com/uploads/1/3/1/6/13161577/tetrix_dc_motor_specs.pdf There have been some part number changes, and motor changes, so you might want to look for more spec sheets based on the motors you have (you can assume that this is close enough, regardless).
A user's page with a fairly comprehensive coverage of NXT motors is here http://www.philohome.com/nxtmotor/nxtmotor.htm (Love this presentation, for FLL kids too.)
http://www.philohome.com/motors/motorcomp.htm has many NXT motors presented (be sure to read the Robut Rules section in Game Manual 1 to see what NXT motors are legal).
Hope this helps!
