what is the point about PAD Flange compared to slip-on? 1

With pad flange you don't have nozzle neck and associated joint stresses. The pad is much stronger, but typically more expensive than a standard nozzle. Further, the pad is a nozzle on its own, the 'slip-on' you mentioned is the flange of a fabricated nozzle, hence there is no direct comparison between them.
Cheers,
gr2vessels

I generally will only use a pad flange if the external projection of a 'regular' nozzle is outside the shipping limits or interfering with something else at the installation site.

Pad flanges are used when you don't want a nozzle neck, for the reasons indicated above and for process reasons such as the reduction of dead zones in the vessel, to avoid plugging of a relief nozzle, or to provide an unobstructed view through a sight glass.