First of all you should never ever use both. Choose either one. Now coming to the recommendation, CDN caching will beat any other server-level page caching mechanism hand down.
This is simply because no matter what kind of caching system you have on your server you can never beat the speed, latency, TTFB of a CDN present in 150+ countries. So, on most cases when the content is cached in Cloudflare CDN, it will be coming from a location which is very near to the user compared to your server which might be sitting on the opposite side of the world.
Also, when you are using server level caching, you are paying all all the bandwidth be it spam bandwidth, DDOS or actual traffic. But with Cloudflare and their unlimited bandwidth – most of the cached content will be served from Cloudflare server and the request won’t even reach your origin server, saving you a ton in terms of bandwidth or server resource cost. Cause even if you have a smaller server running a high traffic site where most pages are cached and coming from Cloudflare CDN, your server will have very minimal load only for the Expired or Uncached content.
I would rather suggest in your cloudflare account you enable Tiered Cache and also get the Cloudflare Cache Reserve option: https://developers.cloudflare.com/cache/about/cache-reserve/
Though it is a paid product but still it is way cheaper and easy to manage compared to paying higher fees for hosting server + complexity of managing the server and it’s internal cache system.
So, whoever looking for getting the best out of cloudflare I always suggest using this plugin along with enabling the Tired Cache option inside Cloudflare dashboard and to get the best of the best cache HIT ratio, if you can get Cloudflare Cache Resrve.
Then you can also take Cloudflare Pro plan, which will provide you unlimited image optimization via Cloudflare Polish + amazing security and firewall.
After this your origin server will only be left for doing the dynamic work like adding content, images, managing comments etc. While most of your requests are served via Cloudflare.
Hope this helps. 🙂
