For me that's hard to compare. They are really different players. As you said, Nowitzki spreads the floor better, which is great for today's game. Malone, however, is certainly more physical, but since you already have Barkley, you aren't lacking there. As I said before, I think both would work with your team, just in different ways. With Malone, you would probably have the most intimidating front court that could be imagined.