Karen,
Each one has things the other has not.
Extras on FMP are 1939 Register and Newspapers. They are very similar for censuses, but I have a slight preference for Ancestry for that because of the search engine.
Since you have already subscribed to Ancestry may I suggest you sign up, but not subscribe to FMP. That allows you to search FMP and get a list of answers. You can then see what you get that is different. To then see the the full records of the different answers you can get credits or perhaps a month subs. Then you will have something to go on next time your subs come up. This method even applies to the newspapers and the 1939 Register , though to drill right down into the 1939 registry you will need annual subscription.
Dave

Hi Karen,

If you're in Sheffield FMP is available free of charge in the libraries albeit without full 1939 data.

If you're not here but still in the U.K. then I would suggest checking your local library website to see which (if any) they offer.

And of course lookout for the regular free access offers.

Good luck!

Nigel