For me it's clear that it is not adlinks. Adlinks are only a bonus, but not the main source of income from adsense. Maybe less than 10% comes from adlink.
Ad units for sure.. if placed properly.. i guess link units pay less per click as compared to ad units.
i think it depends of placement and the near content of the area where you place it, but sure it may be good to bring income
For some reason, my link unit always performs better than my ad unit. I've tried a banner ad above the fold right next to the site title, or below the site title. I've tried the square ad right in the hotspot (top of content, right side). None of those could perform as well as my link ad, which are placed near my navigation.
Ad Unit gives you mony in ONE click link unit may get easy to click because it looks not like AD, but people may just click "Back" and not click the AD