In the context of an *ONE*, an issue that was important for me was that A-class is generally readily available (even at short notice). D availability, OTOH, can be rarer than hen's teeth. (e.g. JNB/SYD).

Now if you're booking well in advance, & won't need last-minute changes, this won't matter, but, IME, "The best-laid schemes o' mice an 'men" often go t*ts-up. (Sorry, Rabbie.)