To me personally, KH 2 easily has the worst battle system out of all entries in the series so far.
I'd say how fast such a gauge for physical attacks would deplete (and how many combos you can pull off with one filled gauge) would depend on length of the combo and the equipped finisher as well. Of course, if you implement such a feature you would also have to think about items to refill the gauge faster (or instantly) like i.e. energy drinks and possibly a new stat (stamina) which influences the overall length of the gauge as well as how much of the gauge is depleted for which attack. This would then of course also be affected by level ups and they could even implement side quests that earn the players awards in form of ability-upgrades which can be applied in order to make attacks consume less stamina (as the character becomes more proficient in performing certain attacks and his/her stamina increases, it should be easier to perform).
For me, the worst battle system in KH comes from KH1 and/or Days, but ... opinions, right?
I do like the stamina gauge idea; however, wouldn't you just end up dodging and running waiting for it to refill too?
Honestly I dont think the problem lies with the button mashing but the lack of any strategy and difficulty. CoM was the only purely strategic kh game because you had to think about everything you do and what deck you use and on proud this added some actual difficultly (especially when your trying to get the cards to break Repliku or Larxenes cards when their so fast)
This series is an action rpg so normal attacks and some slight button mashing is to be expected from it to begin with, the problem is that there are no penalties for button mashing behavior. (unlike CoM were mashing all your attack cards leaves you vulnerable)
While I do agree CoM has the best battle mechanics out of all the games, it's also the most "abusable" (with sleight spamming). With a minimum preparation, even the hardest bosses (like Repliku 4) last less than 1 minute just by spamming triangle from start to finish ...
I think it all just depends on how you want to play the game, and how you want your experience to be.
Now, to answer to the OP:
KHI fire< KHII fire : radial attack, more adaptability.
KHI Ice> KHII Ice : the wide radius makes up for the lack of piercing and the weak range.
KHI Thunder< KHII Thunder : I really didn't fancy KHI's "thunder storm", it seemed (and was) so overpowered compared to the other attack spells. KHII's however was just a single thunder strike (in normal form).
Aero> Reflect : Aero is a cheap lazy spell (for an action game). Reflect, while being cheap, has to be timed right; however, both spells are broken (especially Reflect).
Gravity= Magnet : Gravity is useless in KHI (except on low level runs). I didn't like Magnet much (because it's overpowered), but at least it allowed crowd control, it also didn't work on bosses (which is a plus).