First of all, lets look at what pay per impression and pay per click represents. Pay per impression otherwise known as CPM represents a certain price figure for every 1000 unique visitors to a paticular website. CPM actually refers to the total cost for every 1000 visitors. In other words you get paid a certain amount for every 1000 visitors. Each and every company has their own pay index for every 1000 visitors.
PPC or pay per click is exactly what it says, you get paid every time a unique visitor clicks on one of the google ads on your website or article. Each company also offers different click through prices that vary by topic. With PPC the advertiser goes into an agreement with google or other companies they are affiliated with, to pay every time a unique visitor clicks on their ad in a 24 hour period from the same ISP address.
My personal opinion is that PPC has a lot of disadvantages. First of all it leaves room for the bigger companies to make more money because they can pay out more per click. This in itself is not a fair strategy to the smaller companies who have a limited budget and can only pay so much per click.
The biggest concern with pay per click advertising is that it opens the doors wide open for click through fraud. Click through fraud can be defined in many ways. One way is a person who has a website with revenue producing articles that click on their own ads or going into click agreements with other people. This is totally unfair not only to the advertiser but to the company that hosts the ads. There are always people looking to defraud the system and pay per click advertising offers these people a way to take advantage of the system.
Pay per impression might not be as much as pay per click but it is a guaranteed safe way of advertising. You get paid a certain amount no matter if a person clicks on a single ad. Pay per impression kinda takes a television commercial approach. The ads are their for your convience for you to look at if they interest you. This way the only click throughs will be legitimate and worthy because the person clicking the ad has expressed an interest in the ad or they would not of clicked on it to start with.
Basically, Pay per impression is the safest way of advertisng.