You'll love this.
I sent an email to Sonic Research at 10:31 PM east coast time asking what the cases were made of. Received and email back from Ralf 2 minutes later (7:33 PM their time) telling me ST 122 is polycarbonate and ST 200 is cast aluminum.
To get customer service any better, they'd have to call you before you thought of the question.
+1 The amp top unit (122) is plastic, and has a more detailed, smaller readout (note, tuning mode, etc.) in a separate display (along with the same master strobe circle display of the 200). It does not show the actual note within the 'strobe circle' like the 200 does (I assume they did it this way because the 'plastic unit' is designed to sit on top of you amp using the tuner out of the amp, so they went for a smaller display with more detail). Since the amp top unit is plastic (high quality, but plastic none-the-less), they include a very nice, high quality case to keep it from being beat up in your gig bag... nice! The 122 also has a mic for tuning acoustic instruments, or quick tuning at home when you are noodling with your practice amp or whatever.
The 200 has a large note display right in the middle of the 'strobe circle' that can easily be seen from a distance, and it seems to be built like a tank. The only downside I can see is that you have to remove the four screws and the bottom of the unit to change the battery (versus the easy access battery compartment on the 122), but I assume this was done to increase 'road worthiness'. And, with true bypass, the battery should last for years.
Nice design with both of these IMO. The tracking is really an entire level above any other tuner I've used, especially on the open B.