Both are Onkyo, from different ranges - the engineering approach is shared and the differences are mostly in power, connectivity and price.
| Area | Better | Why |
|---|
| Power | A | Onkyo TX-RZ30 has more power - 100W against 80W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | A | Onkyo TX-RZ30 takes more analogue sources - 6 against 3. |
| Phono stage | depends | Onkyo TX-RZ30 has a built-in phono stage and Onkyo TX-NR5100 does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Room to grow | A | Only Onkyo TX-RZ30 offers preamp outputs - which decides whether you can add a subwoofer or a power amplifier later without replacing the box. |
| Shelf space | depends | Onkyo TX-NR5100 takes up about 15% less shelf area (1491cm² against 1753cm²), and it is 60mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | Onkyo TX-NR5100 costs less - $700 against $1,100 at list. |
| Watts per dollar | B | Onkyo TX-NR5100 is the cheaper watt - about $9 per watt against $11. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo TX-RZ30 B Onkyo TX-NR5100
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
The usual trade-off: Onkyo TX-RZ30 measures better, Onkyo TX-NR5100 costs about 36% less. With most speakers in most rooms that gap is smaller in practice than it looks here. If you play records, Onkyo TX-RZ30 saves you buying a phono stage; if you do not, ignore that row entirely. Think about the whole system: the roughly $400 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.