Final Mix II without a doubt! fighting the organization members alone was worth it. Think about it guys, that 12 extra battles, the insanely hard Terra fight, extra keyblades, Org 13 heartless, Playable roxas battle (which he KICKED MY ASS BTW the first few times) A nice handful of new cutsceens that promote more story growth, a new drive form, the ability to replay the already implicated org 13 fights over and over, and also something alot of people not realize is they actually added MUCH more detailed character models for FM2. The funny part is i never felt compled to buy it or play it since all the text is in JAP, but once i heard about the english patch, i went into overdrive mode to beat it and enjoy the new extras. Totally worth it.
The thing with final mix 1, which a dabbled with in the past is, there arent enough new features to keep you wanting to come back. The only reason it was made is because after the original jap release for the non final mix version of KH1, we got quite a few extras in the english release that japan didnt get. You can kinda think of this as a final mix version although not labeled as such. Thats why it was necessary to make Final Mix 1. It gave JAP what we had PLUS a lil extra which made it worth development. From a US standpoint, there aren't enough extras since we already experienced most them in our initial release.