For me, ad units do better than link units. Of course, it differs depending on the niche and other factors...
I personally prefer ad units over link units.As said by dairyman, it do depends on niche n other factors
i think it depends on the niche... if the traffic is typically your less educated/experienced internet user than the link units work really well...
Link units will give you higher CTR but low earnings ( most of the time ) . Opposite is the case with ad units .
For my sites, ad units work better. But in some pages, link units perform better then ad units. So I guess we have to try and see which one performs better.
I prefer both. Link units perform well in index page especially just below the header. Ad units perform well if it has been placed just below the post or article...
I have 2 on my site, one of each. I think it really depends on the niche, but for me I think if I was a standard internet user then I would be more likely to click on Link Ads, because they don't really look like ads that much. With Ad units it's really clear to me that they are adverts, but some people might think that Link Ads are just navigation, so look less like adverts.