If forced to pick one, i am going with Mike because in our system the FS made his call based on the what the Mike called when looking toward the sideline.
You get to see everything unfold in front of you and your decisions are based off of that. Middle LB's have that to some extent, and are generally who is looked to for play calls but are generally concerned with events 15yds and in.
In todays football it has to be the weakside LB, or Will, or Wham, or whatever the hell you want to call it.
With the MLB only playing 2 downs, most the time, the WLB in most Defenses has become the QB of the D. They are on the field all 4 downs, including Dime packages. They have to know coverages for RBs, TEs, and WRs as well as all the line calls. They have to know the Front Line, Mid Line and Final Line of Defense.
WLB has the most responsibility, therefore I think you should put the smartest player there, assuming the physical aspects are all pretty much equal...