I'm honestly not sure.
That said, I'm secure enough to say that I find significantly more appeal personally in MTFs than FTMs. Maybe I'm a fag myself, but guys doing it with FTMs just seem significantly more gay to me than MTFs.
But the world's a strange place. I've seen incredibly dyke-y dykes who like to have sex with effeminate guys or MTFs, and as much fun as it is to poke jokes at Eddie Murphy's expense, celebs who are rumored to be tranny-chasers never seem to be the ones to get caught -- or really even be rumored rather than joked about -- with cis dudes. Young Thug and John Travolta's closets might as well be made of sugar glass, but I don't think either of them have been known to go for dude-ladies.
The best answer I can give? We're all at least partial-faggots.