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 - 180W against 150W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output barely increases (180W and 150W). |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (95dB vs 95dB). |
| Built-in DAC | A | Both decode on board, but only NAD C 399 BluOS-D handles MQA. |
| Streaming | A | Only NAD C 399 BluOS-D has AirPlay 2, Roon and BluOS. |
| Analogue inputs | evenly matched | The same number of line-level inputs (2). |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | B | NAD C 388 costs less - $1,499 against $2,599 at list. |
| Watts per dollar | B | NAD C 388 is the cheaper watt - about $10 per watt against $14. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A NAD C 399 BluOS-D B NAD C 388
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?
NAD C 399 BluOS-D is ahead, but on only 1 of the measured areas, and it costs about 73% more. That is a lot of money for a narrow margin - NAD C 388 is the sensible buy unless a specific feature above decides it for you. Think about the whole system: the roughly $1,100 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.