I don't have time to write a full reply but two
major points:
- You don't have to pass through either machine; you can "opt out."
- TSA / DHS now uses Z Backscatter Vans (ZBV's) on highway checkpoints, cruising around cities looking in cars and buildings, and some major sporting/concert events. The x-ray levels from ZBV's are presumably much higher than one encounters at an airport checkpoint, there is no "opt out," and indeed you can be scanned multiple times over longer periods of time.