Most servos can only rotate approximately 180 degrees, and I assume that the dimmers are probably closer to 300 degrees. It might be enough that the lights you control only need to go up to 60%, depending on the overall brightness or the lights, or you can have the range set anywhere, such as 10% to 70%, 20% to 80%, etc. You could also try looking for servos that have a greater rotation range (such as the GWS S-125), and there are also some websites that describe how to mod a half-revolution servo to enable it to go for more degrees.
Using the steppers will give you the ability to rotate the full range of your dimmers (and then some). However, steppers "forget" their CurrentPosition if the controller is unplugged or turned off. So you will need to incorporate the last known position into your program or write some calibration routine at start up to determine where the stepper is set to.
I think that the servos will probably be the easiest way to have this all set up.
Alternatively, there are some stage lighting systems that can be controlled by a 0 to 10V input. If your system can be controlled in this way, take a look at the 1002 PhidgetAnalog
which can be programmed for -10 to +10V.