They are:
keyword-targeted content
placement-targeted pay per click
placement-targeted pay per 1,000 impressions
Which one you find working the best for you and why?
Many thanks.
1. run a keyword-targeted content campaign using very specific keyword themes (e.g. product names with model numbers) to target the most relevant content pages.
2. once you have established your metrics and ROI, you can consider a placement targeted campaign for the best performing domains. The main reason (for me) to do this is that it substantially cuts down your campaign management time and you're performance can remain high because your don't get 'dilution' from error pages and parked domains as you do with content targeting. (Google currently does not offer you the possibilty to opt out of those unfortunately)
3. I personally prefer the preferred cost option over the CPM option as it is very hard to predict your costs with the latter. I have found impressions can fluctuate much more than clicks so paying on a CPM basis can be tricky (e.g. a breaking news item related to your campaign can send traffic soaring but is not necessarily going to generate sales)
note: a finely targeted content campaign (using keywords) is in my opinion still superior if ROI is your most important metric.
Hope it helped...