I like the gearing on the T and the front output shaft and shift rails of the J. T low gears are 2.46:1 vs 2.34:1 for the J. J uses no snap ring on front shaft and doesn't tear up the yoke like the T. J rails have better pockets for the balls.
If you can pick up a couple D20's for cheap you can mix and match those parts to build a perfect one, like mine.