Honestly, it depends on what type of defense you're running. If you're using an even front and have a space eater at 1 tech, Donald is infinitely more valuable to your team. Mack would have to play 4-3 end and would still be a great pass rusher but Donald is a generational talent at DT. If you want to play odd front, I'm assuming you'd have to play Donald as a 4 technique tackle/end because he's not going to be a 0 tech nose tackle. Mack can play his natural rush linebacker spot at the 7/9 tech and not have to worry about playing the run or having to face up a much bigger offensive lineman every single play.
Long story short: 3 man front, Mack. 4 man front, Donald.