A very interesting article i found that talks about adidas relationship with the MLS and soccer in North America. The article also touches on how many players will be wearing Nike boots compared to adidas boots on this years Portland Timbers roster. For example, according to the article "Ten of the 30 Timbers wear Adidas-brand cleats. Nike declined to name the Timbers players wearing its shoes, noting that the final roster had not been announced when the company received a question seeking the names. However, through last week, at least 14 Timbers players wore Nike shoes."
However, the majority of MLS players wear adidas boots if they do not have a contract with another boot sponsor which gives adidas the edge in boots on the pitch.