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 |
|---|
| Noise floor | A | Fosi Audio MC351 is the quieter design on paper - 100dB against 95dB. 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. |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Room to grow | evenly matched | The same expansion options on both. |
| Headphones | depends | Only OSD Nero Stream XD has a headphone output. Worth something if you listen late, nothing if you never do. |
| Shelf space | depends | Fosi Audio MC351 takes up about 39% less shelf area (235cm² against 384cm²), and it is 72mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Fosi Audio MC351 costs less - $190 against $240 at list. |
A Fosi Audio MC351 B OSD Nero Stream XD
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?
Fosi Audio MC351 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.