Always after.
Mick, you're on the money. It always seems that once you're on the ground after the p/l, you look around and say how dumb it was to get into the situation of having to p/l in the first place (for non-maintenance issues. Those are kinda handed to you without option). I've been in both- and it's usually the 'it doesn't seem so bad', or 'another minute or two and it might get better'. I've also had countless times avoiding those situations by turning around prior to getting into them- the preferred method. Piloting is a type-A activity, and us type-As don't like having to explain why the helicopter is parked in a field, or we had to pay a landing fee/compensation to a landowner, etc.
As many here have said- it's the wise man/woman that knows where their limits are, and brings that airplane back to the boss. After all, I don't know too many that would honestly be okay with gambling an aircraft and the lives of the crew. -and if that's the case, it's not an operator to work for.
*A side note- help your brother or sister out- if you're having to bring the airplane back, or p/l due to weather, that's what radios are for. The metro types always ask for pireps, especially to verify their predictions of deteriorating weather. I've seen company traffic about to launch into conditions that I'm having to return for, on more than one occasion- using the radio puts it on tape and will make the good weather types run back to give a more accurate forecast and conditions. Again, just like above, experience tells you which ones to stay away from.