I would personally wait on getting the Mustang as many have mentioned in here...let the bugs get worked out. I didn't like the new body style 10-14 Mustangs came out, took a few years for me to warm up to them. That could be that same case with the 15.
Buying a car is emotional and if you are not dead in love with it when you see it and then even more so not impressed when you drive it....then why buy it?
I would keep your 11 and put some cash away and when they start adding some new features and improvements in the next few years, you might be able to walk in with cash and just buy it outright.
My 2 cents.