I don't really see this as an either/or situation. They both have their pluses and minuses. Mo basically won Game 4 against Houston for us when he picked Lin's pocket, saved the ball and then hit the HUGE 3. Will basically won Game 4 against San Antonio for us with his energy and effort.
Mo gets a lot more assists than Will. Mo has the highest AST% on the team, both regular season and playoffs. He has the ability to create easy scoring opportunities or his teammates. Will does not (but Will can create easy scoring opportunities for himself). This made Mo more valuable against Houston, which was a very high scoring series. We needed as many easy baskets as we could get to outscore Houston in that series.
But, Will plays MUCH better on-the-ball defense than Mo. Multiple times he poked the ball away from the Spurs guards in Game 4. It usually ended back in the Spurs hands, or out of bounds off Will, but even though he didn't get the steal, it disrupted the Spurs' offensive flow. And, that's what we need against the Spurs. Force their guards to work and run some time off the shot clock and don't just let them penetrate into the paint and pick apart our defense. Which is why Batum on Parker is such a good idea, and why Barton defending their guards on the second unit works well, too.
I like having both options. It also helps that Will's shot is falling right now (which is why his PER is so high). Both guys are high risk/high reward players that make mistakes and force up some bad shots. For now, due to his length and defense, I hope Stotts continues to give Will burn. He definitely made a difference in Game 4. Let's see what he can bring in Game 5.
BNM