There's several variants of GS6, differing in housing by engine type, torque, TC type, RWD/AWD and gear ratios. So the obvious benefit of GS6-X53 is that you can couple it on ATC500, but taken from X5 or X3 they're to long to bolt on an adapter plate. We've let measured the GS6-X37 from petrol X5 and it was 3 cm or 1.2 inches to long, not to mention the "37"- versions of GS6 are to weak for N62.

So - the best solution for N62 powered X5 is the GS6-X53 gearbox, due it needs much less adaptation work, than RWD-only S6S; in terms of torque, the "53"-versions of GS6 should be even stronger than S6S-420G, which is also strong enough.