The $20 p/m can be cancelled right after you’ve paid to subscribe. However, you will not get any future updates, which I’m hoping will come in quite a lot early after the release, so I would recommend keeping subscribed if you can afford it. If you do cancel though and release a game using UE4, your are still obligated to pay the 5% royalty fees.
Also, the CryEngine vs Unity vs UE4 is kind of up and down in my view. People who used UDK will prefer UE4 straight off the bat, people who use Unity currently (myself included), would obviously want to pay less, but I’m not sure CryEngine would necessarily suit our needs as some people have mentioned it being more complex to use, although they are just peoples opinions.
Also, I don’t believe CryEngine will be doing their new subscription packages till May this year so you could always pay $20 now and see how you find UE4 before deciding whether you would want to try CryEngine. With pricing structures and quality, at the moment I put both engines above Unity.