Public relations:
1) Your job profile is to get free publicity for the company and also promote their products and services.
2) You don't have any control on media over how the media will present your information as they are not obligated to do so.
3) The PR exposure you receive is only circulated once. An editor won't publish your same press release three or four times in their magazine.
4) A third-party article written about your product or service is read by the customer assuming that you are not paid to write positive things about the product.

Advertising:
1) Here you are paid for the ad space and you know exactly when that ad will be aired or published.
2) You have paid for the space so you have a creative control on what goes into that ad.
3) Since you pay for the space, you can run your ads over and over for as long as your budget allows.
4) Consumers know when they're reading an advertisement they're trying to sell a product or service.   _{Your Comment}