My main reason for believing Sean is that he does seem to place a ton of his self-image on "being a gentleman" so I think he's been more careful than most about what he says. He seems to be under the mistaken impression that, "I'm crazy about you!," means the same thing as, "I think you're adorable!" but other than that I don't think he's been all that misleading.
My main reason for not believing him (other than what I saw as a total lack of surprise at AshLee's accusation) isn't that different. I think his image is extremely important to him and it was important that each woman be very invested in the relationship and continue to believe she was "the One" until HE could dump HER. I don't think he would want to risk them seeming over him prematurely (for the show) even if he clearly knew his final pick.
I think the lack of cameras in the FS gave him the chance to say whatever he felt met his needs at that moment, without concern he'd be caught. Worse case scenario would be what we saw: a "he said, she said" unresolvable difference.
Of course, if the rejected F2 confirms that he said exactly the same thing to her (and AshLee's calm "I'm so over you" demeanor made me think she and F2 may have already compared notes about the Fantasy Suite and decided he's a player whom they're well rid of), he's going to have a lot of 'splaining to do!