Different price classes - WiiM Amp Ultra costs roughly 2.9x what Fosi Audio MC351 does. Expect it to lead on paper; the question is whether you will hear the difference through the speakers you own.
| Area | Better | Why |
|---|
| Built-in DAC | evenly matched | Both have a DAC on board with much the same capability. |
| Use with a TV | depends | Only WiiM Amp Ultra has HDMI ARC, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Room to grow | evenly matched | The same expansion options on both. |
| Shelf space | depends | Fosi Audio MC351 takes up about 55% less shelf area (235cm² against 522cm²), and it is 132mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | Fosi Audio MC351 costs less - $190 against $549 at list. |
A Fosi Audio MC351 B WiiM Amp Ultra
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. If this sits under a television, WiiM Amp Ultra is the one to buy - HDMI ARC means the TV remote controls the volume and there is no second box in the way. Think about the whole system: the roughly $359 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.