I think both work fine and don't really have a preference gameplay-wise.
Aesthetically though, I prefer KH1's system. This will sound really basic, but well, KH1's MP has the number of MP bars growing with you and party members throughout the adventure, so obviously there's the sense of characters getting stronger as you progress.
Whereas KH2's MP doesn't really grow in terms of bar size, so it inherently lacks that sense of getting stronger.
And with KH2-onward, party members' HP bars stay about the same size too. So I prefer how it's all set up in KH1, aesthetically. But I understand why they've kept to KH2's look (smaller bars means less visual clutter).