Having used both quite recently, I'm a huge fan of FMP. Their search function seems to be more intelligent (particularly when searching for a date of death) and the results seem clearer. You still get wrong 'matches' but the results just look clearer on the page somehow. You can refine your search and they do match to what you enter.
If you wish to search a particular database, it's very easy to navigate to that one particular area. If you intend to stay with them, they give a loyalty discount. You can also take a 6 month subscription(or used to be able to) so could try them out for a shorter time.
With Ancestry, I always feel you have to wade through reams of completely irrelevant 'matches' and often the correct match has been put into their
'Matches below are less likely to be for your ancestor, but still might be helpful' section despite providing plenty of info to suggest otherwise.
I'm not sure how they compare on cost these days. There's also the fact that they have different records available. Probably most useful on Ancestry at the moment is the Parush records for Lancashire and W. Yorkshire.
It doesn't matter which you go with. You will see a post on a forum or a 'News' release somewhere saying that the one you did not go with is giving access to records you would desperately like to see