One is a streaming all-in-one and the other is an amplifier. Judge them on how many boxes you want and what you already own, not on the spec sheet alone.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 90W against 80W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | A | Arcam SA20 holds up better into a 4-ohm load: its output increases strongly to 150W, where the other barely increases. That matters with demanding speakers far more than the 8-ohm figure does. |
| Noise floor | A | Arcam SA20 is the quieter design on paper - 106dB against 98dB. Audible mainly with sensitive speakers in a quiet room. |
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Streaming | B | Only NAD C 368 BluOS-2i has AirPlay 2, Roon and BluOS. |
| Analogue inputs | A | Arcam SA20 takes more analogue sources - 4 against 2. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier class | depends | Arcam SA20 is Class G and NAD C 368 BluOS-2i is Class D. Class G/H switches its supply rails to cut the heat a Class A/B design wastes; Class D runs cool and draws almost nothing at idle. Modern examples of both measure well - judge by the rest of the page. |
| Shelf space | depends | Arcam SA20 takes up about 14% less shelf area (1580cm² against 1840cm²), and it is 59mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Arcam SA20 costs less - $1,650 against $1,749 at list. |
A Arcam SA20 B NAD C 368 BluOS-2i
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?
Arcam SA20 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. If your speakers are a difficult load - 4 ohms nominal, big woofers, or a sensitivity in the low 80s - Arcam SA20 is the safer partner, because how an amplifier behaves when the impedance drops matters more than its 8-ohm rating.
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.