Both the "labels" were tested by google , and they found that
"Ads by Goooooogle" is often more memorable than "Ads by Google",
so you may notice either label appearing beside your ads at any given time.
Currently, link unit and adsense for search ads are labeled as
"Ads by Google".

The "Ads by Google" label helps distinguish between content and ads,
thus providing a more positive user experience. It also distinguishes Google
ads from other ads so that when visitor click on Google ads, they know they
can trust Google to provide high quality ads targeted to their interests.