2011 Specialized crux comp, 2013 Specialized Rockhopper Pro
My road bike has a setback of 10cm at the moment. I need 11.5cm I think for optimal power and muscle balance but I'll have to get back to that when the season opens up again.
My MTB 29er has a setback of maybe... 8cm... It's not optimal for power or muscle balance and I feel often that my quads are just going to burn off. Almost no hamstring recruitment with so little setback. BUT, as the terrain I ride has basically only extremely technical climbing I need to have my center of balance a bit more forward so I don't wheelie the whole way up. Or actually only one wheelie in these terrains means stop, try to get moving, nope, walk...
I would say that depending on the terrain you are riding it comes down to either handling or power. Flat non technical means power (aka. close to road bike setback) and elevated technical means handling. (whatever works but with a 29er less setback and 26er more)