It makes sense comparing Onkyo TX-RZ3100 vs Onkyo TX-8020 because they both are able to give some premium features.
A significant difference between the receivers in the number of channels, Onkyo TX-RZ3100 has 11.2 versus 2.0 for Onkyo TX-8020. Regarding power, then TX-RZ3100 has such a W/Ohm ratio - 200/6 when TX-8020 has a power of 90/6. The THD is 0.04% for the TX-RZ3100 but 0.03% for the TX-8020.
Characteristics of digital to analog converter (DAC) are different, AK4490 384 KHz/32-bit for TX-RZ3100 and 192 KHz/24-bit for the TX-8020. Only the TX-RZ3100 supports Bi-amping feature. Each of the AV receivers can transmit an audio signal directly to the amplifier and bypasses any DSP processing.
Only the Onkyo TX-RZ3100 can connect to the Internet via WI-FI. Only the TX-RZ3100 has Bluetooth support. The Apple Music service is implemented on considered devices: AirPlay. Only the TX-RZ3100 supports Spotify. Only the TX-RZ3100 provides HDMI signal transmission in standby mode. Both competitors are not able to scale the HDMI signal. Dolby Vision technology found support only on the TX-RZ3100. Only the TX-RZ3100 supports HDMI Audio Return Channel (ARC). Unfortunately, HDMI eARC is not available on monitored devices. Of the two receivers in our comparison, HDMI CEC is present in only the TX-RZ3100. These compared receivers have a built-in phono stage for connecting a vinyl player. A voice control is not available. An ECO mode is not present in each of the AV receivers. Unfortunately, the setup assistant is not available in each of the models.
Only the TX-RZ3100 has a Dolby Atmos multichannel audio format support. A surround technology DTS:X is supported only by the TX-RZ3100.
The Onkyo TX-RZ3100 is better receiver in this comparison. Check Onkyo TX-RZ3100 price