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. |
| Speaker load rating | A | Onkyo TX-RZ70 is rated down to 4 ohms against 8, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | evenly matched | The same number of line-level inputs (4). |
| Phono stage | depends | Onkyo TX-RZ70 has a built-in phono stage and NAD T 758 V3i 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 | evenly matched | The same expansion options on both. |
| Shelf space | depends | NAD T 758 V3i takes up about 17% less shelf area (1836cm² against 2205cm²), and it is 84mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | NAD T 758 V3i costs less - $2,299 against $2,900 at list. |
A Onkyo TX-RZ70 B NAD T 758 V3i
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-RZ70 measures better, NAD T 758 V3i costs about 21% less. With most speakers in most rooms that gap is smaller in practice than it looks here. If you play records, Onkyo TX-RZ70 saves you buying a phono stage; if you do not, ignore that row entirely.
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.