Pose this question online, and you get 436,219 people's opinions on 11,694 different web sites on why the salt (no! the pepper!) shaker has more (no! fewer!!) holes and worse, they all insist on telling you how they reached these amazing conclusions.
Go to an actual source say, the people who have been making Lenox china since 1889 and they'll tell you that, when there is a difference, the salt shaker has more holes.
I checked my shakers, and the ones with an "S" on them have more holes, and the ones with a "P" have bigger holes. One set has no difference whatsoever, so I'm not sure why the company felt the need to label them with an "S" and a "P." If a marriage is at stake I'd throw in the towel and buy some nice grinders.