Two versions of the same amplifier rather than two different designs, so the question is not which is better built but which set of features you actually need - and what the difference costs.
| Area | Better | Why |
|---|
| Power | evenly matched | Effectively the same output - 72W against 72W into 8 ohms is under 1dB, which is not an audible difference in loudness. |
| Built-in DAC | B | Rega Elex Mk4 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Analogue inputs | B | Rega Elex Mk4 takes more analogue sources - 5 against 4. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | Rega Elex-R costs less - $1,395 against $1,895 at list. |
| Watts per dollar | A | Rega Elex-R is the cheaper watt - about $19 per watt against $26. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Rega Elex-R B Rega Elex Mk4
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?
Rega Elex Mk4 is ahead, but on only 1 of the measured areas, and it costs about 36% more. That is a lot of money for a narrow margin - Rega Elex-R is the sensible buy unless a specific feature above decides it for you. If you already own a DAC or a streamer, the built-in one is money you are paying twice for, and Rega Elex-R becomes the more rational buy. Think about the whole system: the roughly $500 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.