I'll be "that guy" to mention that if you're sensitive to temperature, in my experience flying both products, AA keeps their planes relatively cold while JAL keeps them quite warm (uncomfortably so for some). I'm fairly amenable to different temperatures, but I've found myself lightly sweating on JL flights. I've taken other Americans on JL flights and they've been full fledged sweating and miserable. Meanwhile, you'll find many passengers perfectly happy and still wearing thick outerwear during these flights. Generally speaking, even on land in Japan, things are just warmer. In the Summer and Winter, things are just generally always warmer (a lot of heater use, less A/C use). Most foreigners (including from Europe) will say everything is cold in America, during all seasons (too much A/C, not enough heat). Effectively, people acclimate to different standards, and I've experienced that difference flying JL. JL's web site says they aim to keep climate at 24C (75.2 F), but you can find posts here about people saying their flights were at 28C (82.4F). Both of those temps are notably higher than you'll find on an AA flight.

Hard and soft product are generally better on JL, but if you get a good crew on AA, they can be exceptional and while the JL crew speaks great English, some find it easier to converse/banter with the AA staff. I speak conversational Japanese (not great but good enough to hack it), I still find good AA crews are easier to compel to make sure your glass is always full

I'd go with what's cheaper and more convenient schedule wise unless you have a really strong preference for something unique about each airline. To the earlier point, if LPs matter to you, pay close attention to the differences in fares and fare classes. This website can assist in determining maximum LP: https://lpcalculator.com/#/calculator