Have use both sites, but let subs lapse on both so only sometimes got a sub to both. The main reason I use FindMyPast is because thay have Northern Irish Newspapers and don't stick military records in Fold3 though their sub is dearer to start with.
I would certainly not say I have never wished for anything from Ancestry and I agree with 'thebounder' that I find Findmypast's option to sort by columns very helpful, not just by forname but is great to be able to have a 5 year window ordered oldest to newest or sort by county or place or age etc. Is something I fed back to them some years back.
Not bothered about a tick to say have viewed the page as when scrolling through census images or newspaper deaths or looking for someone else things get ticked that you a year or more later find you actually have a spouse on. (scrolling to the beginning of a set of images using thumbnails is easier on Ancestry).
I find Findmypasts Advanced search options generally better than Ancestry's to narrow down within collections eg for Ireland easy to specify specific Catholic Parishes and not resort to having to try location keywords which sometimes assist and sometimes are not in the index so useless so I search there in preference and can do so as is one of their free collections (sub on Ancestry but both used NLI images and unusually jointly indexed).
The ability to specify eg an Irish county and it produce results from all registration districts within is useful, one search instead of several seperate keywords for each district.
I find Newspapers.com USA, Canada marriages and death indexes hard to narrow successfully.
I see what Coombs is saying had similar only yesterday looking for an early Army record (Fold3 on Ancestry) but at least when sort by year can then skip to page X & find them. When using fornames only or Wildcards with few letters of surnames is great to be able to sort and skip to the surnames or fornames starting with a particular letter, on Ancestry have to visually scan each row of 50 results then the next page etc. Many of the Advanced search features on FindMyPast are only available when narrow down using the Catalogue.
https://www.lostcousins.com/newsletters2/midnov21news.htm#MasterclassFMP but collection options differ on Ancestry too.
I like the fact FindMyPast tend to explain the particular record sets in detail, more so than Ancestry where are on both sites.
However, both have pros and cons and as there are transcription errors on both, or a page skipped, is useful sometimes to eg try a search even without a sub on FindMyPast as there will generally be enough info indexed for free to then narrow down and find the same on Ancestry if have failed there.