One area where Donald probably had an advantage over Steyn is that he bowled to better quality of batsmen who had the ability to bat for days. But then Donald vs attacking Tendulkar of 1996 and Steyn vs a mature Tendulkar of 2010 would be tricky to compare.
Steyn could crank up his speed to 155 kph in a test match if he wanted to and he did bowl such spells to Clarke and Hussey, but he found his niche in bowling at around 85 mph with late outswing.
Anyways, I mentioned it earlier, I prefer watching Donald and I don't think much about stats. His bowling in 1992 World Cup was a treat to eyes.
Personally I don't like Donald much (abused the batsman too much for my liking). In fact, in the mid-90s, I would have childish hopes of some young Richards or Richie-Richardson
type batsman turning up and putting this loudmouth in his place in Test cricket.
I am just trying to be objective here. I have to grudgingly admit that he was really good.
I was just talking about you taking those 2 series of Donald as representative examples. Donald was pretty much done at that point, and somehow hung around just to participate in
the 2003 Home World Cup in South Africa (a historic event from South African cricket perspective - first CWC to be hosted by South Africa). Overall, Donald's career stats aren't inferior
to Steyn's (other than Steyn taking more wickets by playing more matches). You exclude Donald's final 2 Test series (both against Australia), Donald's average against Aus becomes 27
and his average against Aus in Aus becomes an even more impressive 23.
Talking about relying only on pace, it makes me wonder though, how many recent fast bowlers were there in the game, who started as genuine express fast bowlers (and were successful),
and as years went on and as their average pace reduced, they were still just as successful based on their skill and guile. Lillee & Akram are the first names that come to my mind.