it seems it's quite likely one of those two will win given what i have read which said, "The Screen Actors Guild’s choice for best actor has gone on to win at the Academy Awards 18 out of the past 22 years" and Denzel won that
but with that said... according to their best predictions they still said they favor Affleck but Denzel is about the only other one who they feel has a reasonable chance to win.
If Denzel wins the Oscar, he will join a handful of actors who have won both an Oscar and a Tony for the same role --
Eight performers have won the Tony and later the Oscar for the same role: José Ferrer in Cyrano de Bergerac (Tony: 1947/Oscar: 1950), Shirley Booth in Come Back, Little Sheba (1950/1953), Yul Brynner in The King and I (1952/1956), Rex Harrison in My Fair Lady (1957/1964), Anne Bancroft in The Miracle Worker (1960/1962), Paul Scofield in A Man for All Seasons (1962/1966), Jack Albertson in The Subject Was Roses (1965/1968) and Joel Grey in Cabaret (1967/1973).
Lila Kedrova did it the other way around. She won an Oscar for Zorba the Greek, and 20 years later won a Tony for the same role in Zorba (1964 Oscar/1984 Tony).
"The Screen Actors Guild’s choice for best actor has gone on to win at the Academy Awards 18 out of the past 22 years" and Denzel won that
I don't think there's ever been a scenario like this though, where Affleck won the BFCA, GG, and BAFTA while Washington won just the SAG. So really, who's the real frontrunner in this case? Does winning the SAG by itself trump the other 3 precursor awards? I don't know if that can really be said.