Depends what type of games you enjoy and if you will be playing solo.
Personally I think both will be great, you can get a microsoft game pass trial and try sea of thieves before you spend a lot money on it, I think that will shine with 3 other friends and going around as a group - i'm not sure solo will appeal to a lot of people and while it has matchmaking, you might get decent people or you might get turds. Great world but remains to be seen what long term content it will have to offer.
Far cry is pretty much a known entity but will full story 2 player co-op and slightly more open world. I think solo it will be a solid 6/10 - good fps, some laughs but nothing really that new to the table, if you have a friend to go through it with, I think it will be a 9/10 for some of the crazy stupid missions.