It makes sense comparing Onkyo TX-NR5100 vs Onkyo TX-RZ630 because they both are able to give some premium features.
A significant difference between the receivers in the number of channels, Onkyo TX-NR5100 has 7.2 versus 9.2 for Onkyo TX-RZ630. Regarding power, then TX-NR5100 has such a W/Ohm ratio - 80/8, 165/6 when TX-RZ630 has a power of 90/8, 215/6. The THD is the same and is 0.08%.
Characteristics of digital to analog converter (DAC) are different, 384 KHz/32-bit for TX-NR5100 and AK4458 384 KHz/32-bit for the TX-RZ630. Both the TX-NR5100 and the TX-RZ630 support Bi-amping. Each of the AV receivers can transmit an audio signal directly to the amplifier and bypasses any DSP processing.
WI-FI modules have the same characteristics: 2.4/5GHz. Rivals from our review have Bluetooth support. The Onkyo TX-NR5100 has support for AirPlay, AirPlay 2, and its competitor in our comparison - AirPlay. Spotify can be used on each receiver. The TX-NR5100 can work with the audio stream from Deezer, TIDAL, Pandora, SiriusXM, Napster, SoundCloud, and the TX-RZ630 can receive a content from Deezer, TIDAL, Pandora.
HDMI signal transmission in standby mode is implemented in each of the devices. The TX-NR5100 can scale the input HDMI signal, unlike the TX-RZ630. The HDR standard - Dolby Vision is supported by these receivers.
The Onkyo TX-NR5100 has 4/2 HDMI inputs/outputs versus 7/2 HDMI connectors of the Onkyo TX-RZ630. The HDMI Audio Return Channel (ARC) feature supports both devices. The HDMI eARC is available only on the TX-NR5100. The Consumer Electronics Control (CEC) feature is present in most modern AV receivers and the models in our comparison are no exception. The TX-NR5100 supports the protection of digital content of the standard HDCP 2.3, versus HDCP 2.2 in the TX-RZ630. These compared receivers have a built-in phono stage for connecting a vinyl player. The TX-NR5100 supports voice control via Amazon Alexa, Google Assistant, Siri, Josh.AI, and the TX-RZ630 - Google Assistant. An ECO mode is only available for the TX-NR5100. Both TX-NR5100 and TX-RZ630 can be configured using the Setup assistant.
Both devices have support for Dolby Atmos multichannel audio format. The surround sound technology DTS:X is supported by devices under review.