PM or Passing?
PM and Passing are good secondaries for an offensive winger.
Passing adds to your wing attack rating (not to the middle). There is a rule of thumb that states that about 3-4 levels of passing = 1 level of wing skill.
Wingers are modest midfielders too and when they are played offensive, defensive and normal, they use 50% of their Playmaking skill (along with their stamina of course).
Defense could be a consideration when played normal or defensive, but a winger played offensive has their defense reduced a lot and their wing skill increased, so for an offensive winger, it's not worth paying for.
So what secondary is best for you?
If you are high-up the food chain and salaries are an issue, good passing on a winger becomes a cheaper alternative than 1 more level of wingskill to generate the same wing attack.
If you plan to play AOW, then passing could help your rating while at the same time helping your wing attack.
Now, If you are going to play any midfielder offensive (IM or Winger), you must ask yourself: "Do I have enough defense behind him?" or "Do I care about defense at all?".
If you have enough defense or if you are playing all-offensive (ie. You don't care about generating defense from your midfield), then passing becomes a more attractive option for you.
However, If defence is a concern, then having some PM on your offensive winger becomes very attractive. Increased possession reduces the need for defense on your team overall.
As your winger is played offensively, their 50% PM contribution can effectively help your defenses by ramping-up possession. The other benefit of PM on your offensive wingers is that they also generate chances that their offensive skills can exploit. So, If you are a team that likes to play 3 defenders at the back, then having wingers with PM becomes of great value to your team.
If you are higher up the food chain and where salaries on your IMs are very high, then having PM on your wingers can be an inexpensive way to help your midfield further.