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 | A | Onkyo TX-RZ70 has more power - 140W against 110W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Analogue inputs | B | Marantz Cinema 50 takes more analogue sources - 5 against 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Marantz Cinema 50 takes up about 20% less shelf area (1759cm² against 2205cm²), and it is 107mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | evenly matched | Priced within a few per cent of each other. |
A Onkyo TX-RZ70 B Marantz Cinema 50
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?
Neither runs away with it - they trade advantages. Let the "depends" rows and the speakers you already own settle it.
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.