Simplified
If you are running on a small track with short run ups to jumps you need bottom end tourque so stay with the 13/50
If you are running on mid size track go with 13/50 or 13/48
If you are running a large open track go with 13/48 or 14/48
If you have a powerful motor on a large track try 14/47
I've tried and run on all the above with my 8T and it all depends on track surface, track size, Engine power output, and clutch setup.