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 - 100W against 100W 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 increases strongly (160W and 160W). |
| Noise floor | evenly matched | Much the same signal-to-noise ratio (110dB vs 110dB). |
| Built-in DAC | B | Both decode on board, but only McIntosh MA5300 handles DSD. |
| Streaming | A | Only McIntosh MSA5500 has AirPlay 2, Chromecast and Roon. |
| Room to grow | evenly matched | The same expansion options on both. |
| Price | B | McIntosh MA5300 costs less - $6,000 against $8,000 at list. |
| Watts per dollar | B | McIntosh MA5300 is the cheaper watt - about $60 per watt against $80. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A McIntosh MSA5500 B McIntosh MA5300
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?
McIntosh MA5300 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 $2,000 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.