I voted for the ad unit, but the link unit if placed properly could get a lot of clicks too. If you don't use the link unit, you're leaving money on the table as they say
Ad units have always made me much more money (when I used AdSense, that is!). I liked them because they were more customizable.
You know, ad unit gives me more money but not much more. Links unit are also very good option especially for sites with not much free space. I always use link unit under title and this gives me much money.
We use both. It helps mix up the content a bit... Looks like too many ads when we used 2 ad units... but when we tried one ad unit and one link unit... it looked more interesting and the variety of ads seem to increase our overall earnings.