This is just my personal experience. I have subscriptions to both Ancestry and FindMyPast
I think that Ancestry is the best site to start with. If I had to choose between them, then I’d probably go for Ancestry and use FindMyPast (FMP) at the library as needed to supplement it.
When I've had trouble tracing a relative on Ancestry I've found that FMP has been an invaluable resource in finding their details. Also, the fact that FMP includes access to newspapers has made a real difference.
But, having said that, Ancestry and FMP do have their different strengths in different parts of the country.
Most of my ancestors come from Gloucestershire and the surrounding counties. Ancestry is great for that area and FMP is just average.
In contrast, I do have one branch of my family that comes from Cambridgeshire and Ancestry is really not very good at all there. But this is an area where FMP really does shine.
So it really does depend on what part of the country your ancestors come from.
I've also found a number of areas where Ancestry are simply missing parts of different censuses, but they are available on FMP.
For example, Ancestry is totally missing parts of the 1861 and 1871 Censuses for various parts of Gloucester that are available on FindMyPast.
In 1861, some relatives of mine were living at the Collier's Arms in an area called "The Island" in Gloucester and in 1871 others were living behind the Royal Oak, on the same street. Others were recorded living on St Mary's Square.
The 1861 and 1871 Censuses for that part of Gloucester are available on FMP but not on Ancestry. Ancestry simply don't have them for some reason.
There are also similar examples that I am aware of with the 1851 Census in Highnam, and the 1891 Census in Redmarley D'Abitot being available on FMP but not on Ancestry.
These are just some small areas that I have looked at and found Ancestry lacking the census records. I am sure that these will not be the only missing censuses - I can't believe that I came across the only missing ones.
I have also found a lot of records relating to convicts being transported to Australia and also World War One service records (and some World War Two records as well) that are on FMP but not Ancestry.
So, overall, I would say start with Ancestry but if you have trouble locating records on Ancestry then FMP will usually be the place to look.