I think each has its merits and detriments, so I use both. I like the fact that FMP will list variations of names much better than Ancestry does. But I'm peeved that FMP doesn't list the birthplace on the index. So you can get a long list of John Kelly's with the registration district but not the birthplace. End up hunting and pecking to find your person.

Both have transcription mistakes, though I think FMP has fewer. FMP claims to be very accurate but its not quite up to their claim.

Another annoyance with FMP is that it will time out if the search is too long. Ancestry never times out.

So we live with what we got.
I have even more peeves with the search engine in Family Search but it's free so can't complain too much.