The 2011 model will have a Mr. Fusion to power its Time Machine. With it you'll be able to go back in time and tell yourself not to buy all those earlier, inferior models.
And the 2012 model will let you unbuy the comparitively barbaric 2011 model.
However, the 2013 is for keeps. It requires you to have first bought the 2011 and 2012 models before you can own the 2013.
The 2014 model lets you go forward in time, skipping the 2015 model and directly buy the 2016 model. However, you're then stuck in the post-apocalyptic future.
So, good question. You need to think long and hard before buying the next Mac model.