Different price classes - Sony STR-AN1000 costs roughly 1.6x what Onkyo TX-NR5100 does. Expect it to lead on paper; the question is whether you will hear the difference through the speakers you own.
| Area | Better | Why |
|---|
| Power | B | Sony STR-AN1000 has more power - 100W against 80W into 8 ohms. That is about 1dB, which is a small step in loudness but real headroom on peaks. |
| Speaker load rating | A | Onkyo TX-NR5100 is rated down to 4 ohms against 6, so it is the safer partner for a low-impedance speaker. |
| Analogue inputs | evenly matched | The same number of line-level inputs (3). |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | A | Onkyo TX-NR5100 costs less - $700 against $1,148 at list. |
| Watts per dollar | A | Onkyo TX-NR5100 is the cheaper watt - about $9 per watt against $11. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Onkyo TX-NR5100 B Sony STR-AN1000
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?
Onkyo TX-NR5100 has the edge on paper, though not by much. The rows marked "depends" are more likely to decide this than the score is. Think about the whole system: the roughly $448 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.