I use both sites, ancestry has alot of databases espcially overseas, but some aren't complete and the searches aren't always that good. However it is a very good all round site. With FindMyPast they are adding so many parish register transcriptions and the census records that have been transcribed are excellent as is their search engine. However if you want to search in BMDs it's trawling through each quarter on FindMyPast at the moment.
So both sites have really good points, it really would depend on where most of your research will be.
Shirl