A good number of primary sources are identical on both Ancestry and FamilySearch, although you may find the transcriptions different and therefore can be missed in typical searches. For example, I've found ancestors on one that I couldn't find in the other because of transcription misspellings, and vise a versa.
But each one also has some sources available that the other just doesn't have.
Personally, I find Ancestry's search (and hint) capabilities far superior to those of FamilySearch, and thus well worth the expense. But that may depend on how often you plan on using their resources.
But these two resources are just the tip of the iceberg as to what is available out there, most of which is not (and likely never be) available on any internet resource, free or paid. Don't limit yourself to these two resources alone.
Both have "family trees", as does WikiTree, and there are many errors in all of them. Research carefully.