Best powerbook for MainStage 3

I'm keyboard player looking to ditch the big keyboards and move to MainStage for live work. Completely new to MainStage and key consideration is reliability / stability.

I'm tempted by the new 2015 MB pro 13" with i7 processor upgrade but notice it is actually a dual core rather than the quad core in 2014 macbook pro 15". Question is which should I go for? Does MainStage take sufficient advantage of a quad core processor to go for older 15" with 4th gen i7 quad core... or is the 5th generation i7 dual core in new 13" more than adequate. My requirements won't be too heavy - just 2 separate keyboard sounds on most songs, maybe 3 on some. Possibly some vocal processing also. Any tips appreciated