not much to choose between them. Donald faced far brilliant batsmen
No he didn't.
Let's compare batting lineups Donald and Steyn bowled to.
Australia : Donald had to bowl to the Waughs, early Ponting, Gilchrist, Slater etc. Steyn bowled to late Ponting, Clarke, Warner, Smith. Donald had the toughest ask here, no doubt.
India : In Donald's time, India were a mediocre batting lineup. Only Tendulkar consistently stood up when it came to overseas tests. Azharuddin was a good player, but the other legends like Dravid, VVS etc were not established players. While in 2000s, Steyn had to bowl to Sachin, Dravid, Sehwag, VVS, Gambhir, and recently to Kohli, Rahane. Steyn easily wins here.
SL : For Donald, Apart from Aravinda , not a single truly great batsman in the team. Steyn had to bowl to peak Sanga, Mahela, Samaraweera (beast player at home). Steyn wins here too.
England : Very poor team in the 90s. Thorpe and Stewart were very good but the rest were mediocre. Steyn faced England's best lineup in decades. Cook, KP, Bell, Trott, all very good players. Easily better than the 90s class which had the likes of Hick Ramprakash and Crawley.
NZ: Again, a very very mediocre side. Fleming and Richardson were good but the rest very poor. In Steyn's Era, NZ have had two of their top 3 batsmen of all time: Williamson and Taylor. Steyn wins again.
WI : Both poor lineups with one great batsman (Lara in Donald Era and Chanderpaul in Steyn era). But 90s lineup was definitely still better. So Donald here.
Pakistan : Pretty clear that in Donald's career, which ended in 2001, none of inzi, yousuf and Younis were St their peak. Anwar was world class. In Steyn Era, Pakistan have had peak Younis and Misbah. This is close, so I'll say it's a tie.
Overall, I'd say Steyn has bowled to better batting lineups on the whole. It's easy to forget that while 90s had ATG batsmen in Sachin Lara and Waugh, there were lots of mediocre batting lineups (NZ, SL, England)