A generation apart: Klipsch RP-5103SA is the newer design, against a 2021 model. Newer is not automatically better, but the older one is more likely to be discounted or near run-out.
| Area | Better | Why |
|---|
| Size | depends | Similar cabinet size - neither is easier to fit. |
| Cabinet | depends | Ported vs Sealed. Sealed is tighter and happier near a wall; ported digs lower but wants space behind it. |
| Expandability | B | Klipsch RP-5103SA: Atmos drivers. |
| Price | A | Klipsch Reference Premiere RP 500SA II is cheaper - $450 against $650, about 31% less. |
A Klipsch Reference Premiere RP 500SA II B Klipsch RP-5103SA
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 RP-5103SA is ahead, but only on 1 of the measured areas, and it costs about 44% more. That is a lot of money for a narrow margin on paper - Klipsch Reference Premiere RP 500SA II looks like the sensible buy unless something else about the Klipsch RP-5103SA appeals to you. Think about the whole system, not just the speakers: the roughly $200 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.