You will get some opinions here, but by wanting to wait for the new body & mpgs, you answered your own question.
Real world mpgs for the '15s is not out there yet. Getting a 15 would be more about getting the new style & trim. If you find a great deal on a 14, you will obviously save some $$$.
Can't go wrong either way.
2014 FX4, SCab, Tux. black, 5.0 - 3.73EL