I don't know much about Satine's acting ability, but she seems to be well-trained, and photos I've seen of her prove she's a better physical match for the character than Kirsten Dunst was. I also prefer her singing voice (here she is covering Radiohead's "Creep"), just in case the role requires a jazzy stage performance like the one in Spider-Man 3. Satine has already portrayed MJ in a workshop for Julie Taymor's Broadway musical adaptation of the comic book, and she was reportedly considered for the part of Black Widow in Iron Man 2.
As for Yelchin, following brilliantly acted supporting parts in Star Trek and Terminator: Salvation, he should be up for a franchise all his own instead of playing second-fiddle here. Anyway, next to Garfield, Yelchin kinda has a dorkier look, and I think of him better suited for awkward Peter Parker-ish roles. Well, he surprised me with his Kyle Reese, so he certainly has the talent to pull off an Osborn-type character, yet I think the movie might call for someone who's not shorter, scrawnier, younger and nicer looking than the 6' tall, 26-year-old Garfield (Yelchin is only 5'9" and a baby-faced 21 years old). The Playlist recommends instead one-time Spidey contender Alden Ehrenreich (Tetro), while I could see -- if height isn't a concern -- the blonder Ashton Holmes.
Now that we've got Peter Parker, who would you like to see joining Andrew Garfield on screen? And will it hurt the production if they wind up casting all relative unknowns for the major parts?