Two things here.
One: you need to be sure it has the FSD option and not just the FSD computer (they technically both have that, even if their displays represent it differently). See Question prior to purchase of used Model X.
Two: some people will tell you FSD isn't worth the 7k they used to charge for EAP, but others will tell you FSD is worth every bit of 10k (I don't know if anyone would say the same at 15k). At the end of the day, it comes down to how you perceive the features and their cost comparisons vs competitors. That having been said, I've seen plenty of claims that FSD being included doesn't really impact the used price. So, if you even might want it, it isn't impacting the price, and you'd have to pay 15k to get it after the fact, then that seems like a no-brainer, after due diligence on (one) above.
There are a lot of differences between basic AP, enhanced AP (EAP, not often available, but one might have it), and FSD, but which ones are important to whom will vary, so there's no good way to summarize this. The biggest difference now is arguable the FSD beta, which may now be available to you without jumping through hoops (I've seen indications that it is generally available, but I don't know as I already have it from jumping through hoops), but some people love it while others indicate it's completely unusable.
You also mentioned upgradability at some point. FWIW, Elon has indicated that FSD HW3 won't be upgradable to FSD HW4, and IIRC, there have also been indications that MCU2 to MCU3 won't be done. I'm pretty sure both of the ones you are looking at have FSD HW3 (I haven't paid close attention to the latest and greatest, but I don't think FSD HW4 has been shipping long if at all), so unless the newer one is a palladium with the horizontal MCU (which I believe is a newer MCU that might be upgradable [hard to say when the next generation of hardware doesn't exist yet]), there is likely not much upgradability on either used vehicle you are looking at as the legacy vehicles are now just that.
