Siblings in MartinLogan's Motion series, so they share a family sound. It comes down to size, output and price.
| Area | Better | Why |
|---|
| Bass | B | MartinLogan 20i digs deeper - 46Hz against 100Hz. |
| Amplifier needs | A | MartinLogan Motion SLM XL is more sensitive (94dB vs 90dB), so it goes louder on the same amplifier. |
| Size | depends | MartinLogan Motion SLM XL is easier to place; MartinLogan 20i is bigger, which usually goes with fuller bass. Your room decides. |
| Type | depends | A Passive On-wall against a Passive Floor-standing - a different job rather than a better one. |
| Cabinet | depends | Passive Radiator vs Ported. |
| Expandability | B | MartinLogan 20i: bi-amping. |
A MartinLogan Motion SLM XL B MartinLogan 20i
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 speakers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
MartinLogan 20i has the edge on paper, though the margin is not large. The rows marked "depends" may matter more to you than the score does. If a subwoofer is in your plans, the bass gap largely stops mattering - you would be crossing over above 100Hz anyway, so judge these on the other rows instead. In a large or open-plan room, MartinLogan 20i has the cabinet volume to fill it more easily; in a small room, near a wall or at desk distance, MartinLogan Motion SLM XL is the easier speaker to get right.
And remember what a spec sheet cannot show: how each looks in your room and whether the finish suits it, what dealer support and warranty are like where you live, and how they actually sound to you. If you can hear them first - ideally with your own music - that will tell you more than any number above.