These are not the same size of amplifier: NAD C 388 BluOS-2i has roughly 3x the power of NAD C 338. That is about 4.8dB, so it is a question of room size and how hard your speakers are to drive, not of one being the better amplifier.
| Area | Better | Why |
|---|
| Power | B | NAD C 388 BluOS-2i has more power - 150W against 50W into 8 ohms. That is about 4.8dB, so it plays usefully louder before it runs out and has more in hand on peaks. |
| Room size | depends | We rate NAD C 338 the better fit for desk or near-field listening, a small room and a mid-size room, and NAD C 388 BluOS-2i for a large room and a very large room. Match the amplifier to the room you actually have, not the one you want. |
| Grip on hard loads | evenly matched | Both behave much the same into 4 ohms - output barely increases (50W and 150W). |
| Noise floor | A | NAD C 338 is the quieter design on paper - 98dB 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. |
| Streaming | B | Only NAD C 338 has Chromecast; only NAD C 388 BluOS-2i 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. |
| Shelf space | depends | NAD C 338 takes up about 27% less shelf area (1349cm² against 1840cm²), and it is 113mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | A | NAD C 338 costs less - $799 against $2,249 at list. |
A NAD C 338 B NAD C 388 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?
NAD C 388 BluOS-2i is ahead, but on only 1 of the measured areas, and it costs about 2.8x the price. That is a lot of money for a narrow margin - NAD C 338 is the sensible buy unless a specific feature above decides it for you. Start with the room. In a large or open-plan room, or with speakers that are hard to drive - low sensitivity, or an impedance that dips - NAD C 388 BluOS-2i has the headroom, and headroom is what stops an amplifier hardening up on peaks. At desk distance or in a small room, 50 watts is already more than most people ever use, and the extra 100 watts would be headroom you pay for and never hear - we rate NAD C 388 BluOS-2i poorly for near-field use for exactly that reason. Think about the whole system: the roughly $1,450 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.