Unless your family tree is huge, or you are researching more than one tree, then a 6 month sub to FindMyPast should be all you need for the 1911 census. After all, going back to 1911 will mean grandparents or g. grandparents to most of us, and you've only got 8 of the latter. I took out a 12 month sub to FindMyPast, and (to be honest) I think it would have been cheaper for me to use PPV, because I've only used it about 30 times (although I did pay for about 10 records by PPV before yearly subs were available).
In my humble opinion, the searches on Ancestry are miles better than FindMyPast - I proved this at the weekend when I typed my g.g. grandfather's details into the search engine of both. The Ancestry search came back with all the census records of those matching my details, along with full BMD lookups, marriage records and banns (he lived in London), and links to other people's trees. On FindMyPast, all I got was a list of records that I could manually wade through. No contest, as far as I'm concerned.