I feel like this is less clear cut than it's made to be. It heavily depends on how you define "best player."
If the question is "who is most likely to lead the team in scoring?" Andrej is the safest and likeliest bet.
If the question is "who leads the team when we add up pts / rebs / ast?" Mirk is the safest and likeliest bet.
I really don't think either guy will need to be too much different from what they were last season. Just continue to get better at the things you're good at.
Andrej's increased points will come from increased PT (only 26 minutes / game last season). His points per minute and points per possession were elite. Add in a improved shot, he could be one of the best scoring options in the B10.
Mirk has the best all around impact on this team, and it's not close.