Different kinds of product: one is active with its amplifier built in, the other passive and needs one. Judge them on how you want to own the system, not just the specs.
| Area | Better | Why |
|---|
| Bass | evenly matched | Much the same reach (62Hz vs 62Hz). |
| What you buy | depends | Klipsch R-15PM is a complete system - just add a source. Klipsch R-51M also needs an amplifier: more money and another box, but you choose it. |
| Connections | depends | Klipsch R-15PM has Bluetooth and phono built in. With Klipsch R-51M that all comes from the amplifier you pair it with. |
| Upgrade path | depends | Klipsch R-51M can be re-amped later and outlive several amplifiers; Klipsch R-15PM ages as one unit. |
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Expandability | A | Klipsch R-15PM: a sub output. |
| Price | B | Klipsch R-51M is cheaper - $359 against $499, about 28% less. Speakers only - an amplifier narrows that gap. |
A Klipsch R-15PM B Klipsch R-51M
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?
Klipsch R-15PM suits you if you want one box, ready to play, with nothing else to buy. Klipsch R-51M suits you if you already have an amplifier, want to choose the electronics, or expect to upgrade in stages. Neither is the better speaker in the abstract. Think about the whole system, not just the speakers: the roughly $140 saved by taking the cheaper pair would buy a subwoofer or a better amplifier, and either usually does more for the sound than the difference between these two.
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.