Both CloudFlare and Google are for-profit corporations which want to take actions to maximize their profits. Insomuch as they are profit maximizing, we should expect them to take actions where the expected gain is greater than the expected cost, and to prioritize actions which have the maximum gain over actions which are only barely profitable.
If the cost -- in terms of extra labor and reputation if the data-selling becomes public -- is estimated to be less than people are willing to pay for the data, why would any profit-seeking corporation choose not to sell your data?
We can't trust companies not to sell poisoned food, even though that's a huge reputational hit. We can't trust companies manufacturing herbal, vitamin or nutritional supplements to actually put the herb, vitamin or nutritional factor they claim they are in the bottles they sell. We can't trust the makers of USB cables to produce cables that actually meet the USB specifications. Why should we believe that any corporation, regardless of whether or not you pay them for their goods and services, would leave the money to be made selling your data on the table, even if there's a potential reputational hit if the practice becomes public?
But Cloudflare's business model is different: they get rich by making the internet faster. Plus, it's great publicity for them. Their customer acquisition focuses largely on winning developers (by letting them use a great, barely cut-down-at-all service for free) so they use it in companies later. This helps with that a lot.
CloudFlare doesn’t make their money off of brokering my attention, they have a decent track record of doing the ‘right’ things (or at least not the ‘wrong’ things), and they’ve made some decently pro-privacy statements in the past.
It seems that everyone wants to collect data on everything and figure out how to sell it off, so I’m not putting it out of the realm of possibility that CloudFlare could do shady stuff and abuse my privacy as well- but their general line of business doesn’t require it the way Google’s does.
All things otherwise equal, I’m gonna trust the company who’s business isn’t selling my profile a bit more for most things. I used to use Google DNS a lot, now I use CloudFlare’s. I trust them both more than Comcast, AT&T, and Verizon with respect to technical competency and security at the very least.