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 |
|---|
| Built-in DAC | B | Both decode on board, but only FiiO AIR AMP handles DSD. |
| Analogue inputs | evenly matched | The same number of line-level inputs (1). |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | Fosi Audio MC351 costs less - $190 against $300 at list. |
A Fosi Audio MC351 B FiiO AIR AMP
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?
FiiO AIR AMP is ahead, but on only 1 of the measured areas, and it costs about 58% more. That is a lot of money for a narrow margin - Fosi Audio MC351 is the sensible buy unless a specific feature above decides it for you. Think about the whole system: the roughly $110 saved by taking the cheaper one would buy better speakers or a subwoofer, which does more for the sound than the difference between two competent amplifiers.
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.