Mike Young writes:

The discrete nature of the step pulses and
the encoder stream would seem to be inherently noisy, but is easily
averaged out in steady state motion.

Dunno what you mean by noise. There is no noise of any consequence.

OTOH, acceleration and
deceleration would seem strongly dependent on the controller being
able to maintain sync, rather than solely on the motor's available
torque as is the case with a stepper.

Dunno what you mean by this, either. Whatever the motor is capable of
doing, the controller and software will actuate.

So, how much can I expect to pay for a 400 oz-in servo?

You can get a new 1/3 or 1/2 HP PMDC motor surplus, and retrofit an
encoder, for less than $100. Ready-made DC servos are more expensive.

http://www.truetex.com/servomod.htm
http://www.truetex.com/mcgpd34002.htm