I do NOT recommend getting a 13" model. In addition to reading many posts on different forums about audio/CPU issues when Mainstage is run on a 13" machine, I've also witnessed these issues firsthand on several 13" machines. In every instance, there were massive CPU issues and audio problems, especially when MainStage is in Perform Mode. The CPU usage in Perform Mode was literally 50-70% MORE than Edit Mode...I couldn't believe it, and it occurred on different machines running both the same and different MS concerts, and with varying audio interfaces. Some tried running MS in Low Resolution Mode (which is pretty ugly) and others have tried using MS in Edit Mode instead. I have a couple of friends who bought 13" machines specifically to run MainStage at gigs (because the 13" models are cheaper) and after weeks of frustration, they either sold those machines or they ended up being used for non-music purposes. Personally, I believe these issues are due to the 13" models not having a discrete graphics card and because MS is so graphically intensive, the CPU simply cannot handle it.
I highly, highly recommend getting a 15" model, preferably with 16GB RAM and an SSD.