I think I, the person, will open the dishwasher door and interact with its contents more frequently than any base cabinet. But any cabinet that has stuff in it is being used perpetually.
So I think the question is where is the best equilibrium of value; volume of dirty dish space or volume of clean stuff space.
The way I see things now is that my family of three produces one drying rack worth of hand washed dishes per day, which is a smaller volume than an 18inch dishwasher.
On the other hand, the layout with the 24" washer and 90 degree cabinet configuration wastes less space than the 18" one at a 45 degree angle, but it also has less aesthetic (and ergonomic) appeal.
Another consideration is the cabinet design is hugely simplified in the 24" option, and as a “beginner” “cabinet maker” (ie someone who will do it exactly once in their life), that might be a less risky bet.