Be careful when you’re split testing ads.
In the screenshot below, when we look at the red boxes, it looks like the first ad has the higher CTR.
But when we segment by “Top vs Other”, we see that the second ad has a better CTR in the blue boxes.
The first ad has a higher overall CTR because it’s been shown more often in Top ad position than on the side (compare the impressions for Top vs Other).
It’s still early days for this test, but my money is on the second ad.
(Oh, and knowing which device your impressions and clicks are coming from can change this too. In the case above, I know this is all non-mobile traffic. If you’re ads are showing on all devices then you’ll have to download the data and segment by “Top vs Other” AND by “Device”.)