Both have a highly chances to join.
Viewing the chances in a rational perspective: Carrot have more development with the crew, but it lacks of having established a relationship with Luffy. In addition to the fact that in the last chapters, her priority is to get revenge, instead of protecting the interests of the Mugiwaras (Luffy reach the top or saving Momo).
Viewing the chances in the Oda perspective: Yamato seems like a character only introduced with the purpoise of joining (Like Brook, Franky, Chopper). His introduction is like, "hello im waiting to join you". And instantaneously she makes a friendship with Luffy, even he order her, one of the most important mission on Wano raid (protecting Momo).
And in a few chapters before, Oda over-wanking Oden. Introducing a characters who remembers all the greatness and love who Oda haves to Oden (some users in the forum dislikes this, but in the end they know the truth of the purpose of Yamato, even if they denying).
My bet is on Yamato, but if in the end the 10 mugis thing not ocurs, obviusly both joining.