I definately think a website is more stable for the long term, especially if you accumulate repeat visitors over time (that is the kind of sites I like to focus on).
As for PPC, yes the programs that you are running can die out quickly, but I think that the long term strategy here is your knowledge, and your diversity. If you know how to create successfull PPC campaigns, you will always beable to adapt to change and create new ones.
Think of it this way... If you have 5 websites vs 50 PPC campaings. You will probably spend more time setting up your five websites, yet they are more susceptible to changes in the search engines because you only have 5. But, with your 50 PPC campaings that took you less time, if 15 of them die out, you can create 15 more (using your knowledge).
From my experience, you will only lose one or two at most at time. This give you plenty of time to start up new campaigns to take the place of the "dead campaign"