I personally put Ace over Croc power wise if we involve Haki use but back then before it was a thing I think their powers would stalemate each other unless Ace can put out enough heat to turn Crocs sand into glass and make him helpless in a fight.
Smokers fruit put out Ace’s flames so Croc’s sand would do the same. Haki wasn’t a factor at all in Alabasta so you can’t really involve that in fights that early in the series.