It really depends on what you need as a team. Of the two, I'd take Mercer on this team. We need his scoring.
Yeah, but MKG gives you Garnett-level intensity and pressure on teammates to perform, the breakfast club, etc, minus the latent serial killer tendencies of KG.
MKG could get Murray to stop hot dogging and might've gotten Skal and Poythress to step up the intensity sooner.. and the defensive effort would be up across the board.