My advice would be to make a list of the counties/areas of interest that you seek and then visit the two sites and view the catalogues/collections and see which covers those areas the best.
From personal experience Ancestry are good for Lancashire whilst FindMyPast are better in Shropshire.
I'm led to believe that Ancestry has the better coverage of London but it could depend on which areas you need.
Both have some Scottish records BUT transcripts only. ScotlandsPeople is the only place for Scottish Records if you wish to see images.
I have no experience of Irish records so will leave that for others to comment.
A lot of information can be found of
free sites like:-
FreeBMD an index to Births, Marriages and Deaths in England & Wales from 1837 to 1983
FreeReg covers Parish Records for Baptisms, Marriages, and Burials but coverage is patchy.
FreeCen covers the Census taken every ten years. dates from 1841 to 1901/11, again coverage is patchy.
The transcriptions are done by volunteers, so all are works in progress.
Ruskie mentions a free trial on Ancestry, It is actually a free weekend's access to COMMONWEALTH RECORDS from Friday 30 March to Monday 2 full list of records is here
https://search.ancestry.co.uk/search/group/commonwealth_records?o_xid=86746&o_lid=86746&o_sch=Email+CampaignsIf you have to sign up don't forget to uncheck the renewal box or check the cancellation box, whichever is present, to stop auto-subscription renewal.