I'd go with
JuicyAds because I can pick individual sites where I want my ad. I can choose based on % of english speaking traffic, # of impressions and/or clicks of current/past ads, etc. Each site in their system gives me info like how many clicks the top ad got and what the average is, % of countries and the cost per click for the ad spot.
With JuicyAds, I can also take out a run of the network ad for a specific cost per click.
With the individual sites, I pay a flat fee and if I get more clicks than I hoped for, I don't have to pay more.
Some sites set their rates as low as .009 per click which gives them low flat per time fees.
I can place ads in many various sizes of image as well as text