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 | evenly matched | Effectively the same output - 120W against 140W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Analogue inputs | A | Onkyo TX-RZ50 takes more analogue sources - 6 against 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Onkyo TX-RZ50 takes up about 14% less shelf area (1892cm² against 2205cm²), and it is 72mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Onkyo TX-RZ50 costs less - $1,700 against $2,300 at list. |
A Onkyo TX-RZ50 B Onkyo TX-RZ70
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?
Onkyo TX-RZ50 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Think about the whole system: the roughly $600 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.