OP, here is my $.02.
I went with the 2013 STI because I am in love with the exterior looks and performance. I briefly considered waiting, but the 2014 will be identical to the 2013 (so I have been told). The 2015 will be a new model year with many differences from the Impreza WRX/STI line (again, so I have been told). My feeling is that it's never a good idea to purchase the first year of a new model (manufacturer hasn't worked out all the bugs).
If you love this model, don't wait. That way would won't miss your chance to own something that you love. If you end up falling in love with the new model, simply trade up.