I would say "Yes", it's worth subscribing to both, as I often run them side by side. What can be found in one is not always in the other, and the results are presented in somewhat different ways.
It does depend on what you're looking for. I have found masses of info on FMP which isn't available on Ancestry, as my background is entirely Welsh, and Ancestry hasn't published what I want, yet. Conversely, my husband's ancestrey is 100% English, and he has rarely used FMP, as most of what he needs is on Ancestry.
So my advice is: decide what you probably need to find out, and work with Ancestry for a while, and then you may find you won't need FMP.
The other thing I could suggest: if you know anyone with a FMP subscription and perhaps they could look up something for you on an ad hoc basis.