No single best amplifier here. What decides it is what you have to drive, what you need to plug in, and how much of the system you want in one box.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 120W against 100W 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 | Marantz Cinema 60 takes up about 16% less shelf area (1591cm² against 1892cm²), and it is 73mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Onkyo TX-RZ50 costs less - $1,700 against $1,800 at list. |
| Watts per dollar | A | Onkyo TX-RZ50 is the cheaper watt - about $14 per watt against $18. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo TX-RZ50 B Marantz Cinema 60
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.
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.