Only the "plus" versions are different (has a different clock generator but the rest is similar)

RS-918 ... The basic mcHF radio
RS-918-PLUS ... The RS-918 with an external 6800 mAh battery.  It also has 1024K of sorage space
RS-928 ... The RS-918 with built in batteries
RS-928 PLUS ... RS-928 with different clock (Needs a "modded" firmware for the clock generator)
RS-978 ... The RS-918 with a snap-in battery pack
.....
- The RS-929 and the RS-928 PLUS versions need unique firmware for the clock but
hardware-wise all radios are the same so the spurious signals and the sideband suppression (ghost signals plus minus 12, 24, 36 Khz etc) are same in all radios

- Tests should only be done after carefully set the I/Q correction
- I/Q correction should be done with a S9 signal but not much higher (too high level and you will not find the best setting) ... an S-9 or S-9+10 dB signal should be far down in the noise after I/Q correction is set
- The manual I/Q correction worked a tiny bit better as the auto correction (So I prefer manual correction) ... 
- Only after you have done this (plus switching on the if offset to plus or minus 12 Khz) you can do a meaningful test with a signal generator (to test spurious signal suppression or sideband suppression)