Always Super, never Max.
I feel it'd be better if Super lasted 250 steps and Max lasted 500, but nope.. 200, 250, respectively.
Naturally there's no use in Repels whatsoever in Scarlet/Violet, but in prior games I would usually buy Super Repels in multiples of 10 when I had the money. If I'm not seeking a certain Pokémon or grinding, then random encounters are just too interrupty.