One advantage of the DC drive is it can be used on different mounts by tuning the voltage, whereas the stepper drive is configured for a gear ratio and will only work good on a mount with that ratio; although some people have used them on mounts they weren't designed for by replacing a crystal on the circuit board to adjust the drive frequency.
For basic astrophotography the stepper drive would be much better.
These days it is easy to build your own RA drive controller. A decent motor costs around $22 and an arduino and driver kit around $15. There are many examples of open source controller code for the arduino, which can be adapted for a particular RA gear ratio. The advantage of this setup is that you can use it on other mounts, with just a small software change. The only problem is that you have to make your own brackets to mount the motor. It is not trivial, but I did mine with L brackets and a hand drill.
Apologies for my abysmal ignorance in the matter:
1. What is a difference between a non-stepper and a stepper motor? Does this relate to Jon's characterization of some motors as "true track drives"?
2. As we're discussing this… I'm still on visual, but might eventually want to dabble in astro-imaging. I think I understood that the Astromaster RA motor would only allow limited exposure time even assuming good polar alignment. Would the dual-axis CG-4 perform better in this respect?
The accuracy of the Astromaster drive is fine for visual. The problem with the Astromaster DC motors for AP is that it is very hard to get the accurate tracking rate needed or AP. I still was able to take 20-30second shots using it for short durations. But a lot of time is spent tweaking the potentiometer.
With the stepper, maintaining accurate speed is much easier. However depending on the mount and the scope it might still be very hard to go past 30s-1 min exposures. This is due to periodic error on the RA axis. You will have to add guiding, so you might as well plan for it.
Options for guiding (on a manual mount) are:
1. Build your own RA drive. ONSTEP is good path to follow, if you are build one of their compatible designs, all the software is available. You get GoTo as well with this setup. You can do a simpler drive without ONSTEP, but connecting it to existing AP tools can be tricky.
2. Some of the dual axis drives, can be easily modded to add a ST-4 guiding port. No GoTo, but you can guide.
3. Get a GoTo dual axis drive with a guiding port. Not sure if one exists for the CG-4, though.