You really need to decide what you want this plane to do for you. If flying is the main concern and you wish to get into the air quickly the GP wins hands down. If you are planning on developing an entry into Scale the Top Flight is the clear winner. It can be made very scale and if you are planning on really persuing the scale end of the hobby this would be a great place to start. It is more difficult to fly than the GP especially if you incorporate all of the scale option into the build. It will however give you first hand flight experience on the way a Scale warbird will fly. You will get to know the rudder and the use of flaps both of which are necessary if you plane on ever taking on a true scale plane. In defence of the GP Mustang it is one of the nicest flying planes out there. Most folks recognize it as a P-51 but it's not very scale. Id make my decision based on where I want to be in this hobby two years down the road.
Jack Devine