How much better is SIE than MIE?

I run an eagles/niners TT.   Vernon Davis is 2 speeds faster than Ertz so I’m using him. 
 

but I see Ertz gets discounted SIE vs the MIE that my Davis gets.  Everyone always says SIE is so much better.    Do I consider replacing Vernon with Ertz?    +2 speed is pretty big