It's not the record against their respective teams which makes people rate Donald lower, it's how they both went about their business. McGrath, even when not taking wickets, you could see the Saffers found him tough to play. Donald against the Aussies, was less patient when they didn't just fall over or be intimidated right away.
To me, Donald's best spell against the Aussies was in '94 on the last day at Sydney. Fanie took 6 but Donald bowled an immense, probing, patient but ****ing hostile spell at the other end which gave them absolutely nowhere to go except get out to Fanie. Instead of seeing more of that in the years afterward, we'd see press about how Donald was bowling quicker and more dangerous than ever and watch out Aussies. Possibly he felt pressure having to live up to that but the result was Donald trying to boss his way into the Aussie batters' heads and when it inevitably didn't work, he'd try to bowl even quicker and nastier and get slapped around instead. Was massively hampered by very negative captaincy from Cronje too.
That's why Aussies tend to rate him lower. His record against them flatters him a bit, really.