Since Joss Whedon would only be interested in directing a film that he has written (and he's supposedly working on Wonder Woman now, anyway), he isn't on the short list of potential directors.
Neither of these guys prior films have exactly knocked one out of the park... both of them having fairly poor ratings for their films if you look at what they've done in the past, whereas Matthew Vaughn had a number of very well received films under his belt. While having Vaughn direct would have been the equivalent of Christopher Nolan on Batman Begins, with a fresh, non-clichéd take on the heroes, I having a feeling we're in for something that will be in the end, mediocre at best.
Combine the following, and you have a formula for trouble:
1. Only 9 weeks to go until shooting begins.
2. A screenplay drafted by a director who is no longer with the production.
3. The fact that whoever comes on board will want to do his vision of the film and not someone elses.
4. Some of the plot points that have come out concerning this film that are marking it as "controversial", as in "don't bring your kids". (Drag your mouse over the area below to read.)
"Three major characters that absolutely should not die, die. Really dead, too. Not Spock Dead. And one mutant who should not lose her powers, loses her powers permanently. Really loses them, too. Not SUPERMAN II loses them. And the sex scene... when you see who does what to who... oh, boy."
5. Two directors to choose from, neither of which have a stellar track record overall.
It just ain't lookin' too good, folks.