(or over MyHeritage or over FindMyPast or over [insert other paid site here])
TL;DR: FamilySearch is free and accessible to everyone.
FamilySearch is a “one global family tree” site (like WikiTree) while Ancestry is an personal family tree site. FamilySearch is designed for collaboration.
That said, FamilySearch isn’t perfect. The search engine is inferior to Ancestry’s when it comes to finding records if the person’s name is misspelled in the record or if the record was transcribed incorrectly. The website also has more bugs and is missing some important record collections that come in handy when you’re researching people in the US, but I think it should be the first place you search for records because the basic ones are there for a lot of people (census, marriage, and death) and it’s important that people be able to access sources as easily as possible. On WikiTree there seems to be an unwritten rule that everyone is supposed to use Ancestry as much as possible because the vast majority of profiles only have sources from Ancestry on them. I don’t update sources other people have added unless they’re wrong and will use Ancestry to follow the profile’s style, but it bothers me.
I don’t avoid Ancestry completely, I use it to supplement FamilySearch research. There is a feature on FamilySearch that will let you search for more records for someone on Ancestry (and a few other sites) by using data from the person’s FamilySearch profile page. If I find a record on Ancestry that is from a collection that FamilySearch also has but I didn’t find it on FamilySearch, it’s usually because (a) the person’s name is misspelled in the original document or transcribed wrong in the record, or (b) the person’s birth or death year in the record is much different than the birth or death year in their profile. I use the WikiTree Sourcer browser extension on the Ancestry record to find the same (or similar) record on FamilySearch (you can use this part of it even if you don’t use WikiTree). If that doesn’t work and it’s a census record, I usually manually look for the record by browsing the actual document scans. The scanned document page on Ancestry is the same as the one in FamilySearch and the census pages have identifiers. If I still can’t find it (because FamilySearch failed to index some records) I will use the Ancestry source. If you decide to use an Ancestry source please also add details about what’s in the record so that people without an Ancestry subscription will still be able to know. Using an extension to create the source citation is a great time saver. I use WikiTree Sourcer but there are others, including some not specific to WikiTree. Please see the WikiTree Tools post for more info.
Note: the 1950 US Census is indexed pretty badly on both FamilySearch and Ancestry. It’s always best to look at these document scans directly. I have also discovered that some people/households aren’t indexed on one site or the other for previous censuses, but it’s a lot more scattered.
If you do use a FamilySearch record as a source, please also link it to the person it belongs to in FamilySearch’s family tree. This may involve creating the person’s profile yourself. I’ve often seen a record used as a source on WikiTree that wasn’t also linked to the person’s profile on FamilySearch. You can also add an Ancestry record to a FamilySearch profile as a source (and vice-versa). Not everyone visits WikiTree, so it’s better to have information on more than one website.