Marshall is certainly the best seamer of the modern era IMO, but Donald is not too far behind either - maybe 5th, 6th-ish.
And you could make a case for Pollock being right up there, too.
Who knows how good Ntini might have been playing all the time with those 2? Maybe better, maybe worse, we'll never know.
Equally, we'll never know how bad\even-better Marshall might've been had he not played most of his career with Holding, Garner, Walsh, Patterson, Benjamin, Gray, Bishop and Ambrose, and Holding and Garner with Roberts, Daniel, Croft, Marshall and Patterson.
Maybe "surely almost as good as ..." would've been a better thing to say, though...