SRK's acting as Don shouldn't even be compared to realistic gangster's performance like Ajay in Once Upon a Time in Mumbai & Company. Difference is that SRK as Don was all about his style, charisma and charm which is what worked. However, Ajay's performance in Once Upon Time in Mumbai & Company was more arty and displayed very well on screen. That's the reason why Ajay look more believable and convincing as a gangster because it was realistic.
I'm not trying to criticise SRK's performance as Don because he was good but what I'm just explaining the differences in terms of acting and role portrayed by both actors.
Ajay's performance was better in both films. I strongly doubt if SRK could ever do both roles the way Ajay had done it.