Congratulations on the choice of 1.2m dish.
I like that one, too.
There aren't many motors actually rated for the big dishes.
Oh, sure there have been projects described on the forum where big dishes have been mounted on small motors, but then we try all sorts of things.
STAB HH-120 and DG-280/380 are about it.
If I had the dish and wanted to motorize it, my money would be on the 380, as the only choice.
That's personal opinion, based on years of monitoring feedback from members on all sorts of choices.
...And the fact that I'd prefer to take my chances on other parameters, not the reliability of the motor.
Since the GeoSatPro 1.2m dish WILL mount on the larger 55mm motor tube, I would absolutely get a motor with the larger shaft.
That would be the 280 or 380, as you prefer.
Gives more mechanical rigidity at a weak/slip point.