My money is on the Lockheed Martin T-50 design. It is already flying in Korea and they are just modifying that design to fit the requirements for the USAF T-X competition. Heard from a reliable source that they are adding air refueling capability to the T-X, which is a complete and utter waist of money for an advanced trainer. Never had it before, don't need it now. Students from all airframes have done just fine during Air Refueling training in their follow on airframes. No need for that capability in an advanced trainer.....
Flying close trail does the trick pretty good to give them an idea of being behind and below another plane.