Frequency resolution (after in-band calibration with the crystal calibrator) and stability are outstanding. Stations are exactly (+/- 400 Hz) where you expect to find them. After warmup, stations stay "in tune" for hours and days at a time. Art Collins knew what he was doing when he designed these radios......
The only caveat for band-scanners is that frequent use of the band change knob can result in "R-390 wrist". While tuning within the 1 MHz bands isn't a problem, changing frequency from 1000 KHz to 1010 KHz requires a band change and running the PTO from 1000 all the way down to 10. This is lots of exercise and an invitation to repetitive stress syndrome. But not enough to keep me from using the radios almost every day.
While working on a friend's R-391, I had the chance to compare the performance of the receivers side by side. The R-391 (and R-390 or R-390A) has more controls than a R-392. They have 6 IF selectivity positions (0.1 to 16 KHz) while the R-392 only has 3 (2, 4 and 8 KHz). They have 4 selectable AGC positions (slow, med, fast, and OFF) while the R-392 only has on (fast) and off. They have audio filtering (narrow, medium and broad) while the R-392 has only broad. The R-392 has a few extra functions so that it can operate with the companion T-195 transmitter. They have both balanced and unbalanced antenna inputs, with the balanced connection providing an extra stage of RF selectivity; they also use connectors that aren't commonly found in the ham shack. The R-392 has two unbalanced connections, one for a normal BNC, the other a spring-loaded terminal for a single wire antenna or lead-in.
There was almost no difference in overall RF/AF selectivity when the R-391 and R-392 IF selectivities were matched and the R-391 audio selectivity set at broad. The R-392 audio selectivity was slightly narrower than R-391 broad, but broader than R-391 medium. In medium and shortwave broadcast listening there was almost no difference in overall performance at the 4 and 8 KHz selectivity positions. In SSB/CW there was almost no difference at the 2 KHz position. Both receivers required riding the RF gain for optimum SSB/CW reception. It was extremely difficult to tell which receiver was selected under many conditions. When the going got really tough, the extra IF and AF bandwidths of the R-391, plus it's adjustable threshold noise limiter, really did make a difference.So if you're on a budget, buy the R-392 and rest assured that you've got one heck of a radio!