DC will get Wonder Woman in her own film before Marvel does for several reasons. First, as far as Marvel doing a Black Widow film, as I understand, Kevin Feige expressed interest in doing the film, and it’s “in development” which really just means they’re seeing when and where it will fit in. As it stands right now, there’s no official plans for it, and the absolute best case scenario for it would be the end of 2017, but much more likely would be 2018 or 2019.
Right now, Marvel has two films scheduled for this year, Captain America: The Winter Soldier and Guardians of the Galaxy. Next year is Avengers 2 and Antman. After that, they’d specified three release dates, two of which are 2016 and one in 2017, but not the specific movies; however, we know that they’ve officially greenlit Thor 3 and Captain America 3, and the current rumors are the third one will be Dr. Strange. Aiming for two films a year, the earliest they have a slot available is then the end of 2017, but since I imagine Avengers 3 would probably hit in 2018, and they’re already taking risks on Antman and likely Dr. Strange, I think it’s much more likely they’d drop a safer property in that slot, particularly to set up for Avengers 3, which I think most of us are expecting to be the climactic battle with Thanos. I could see them dropping a Black Widow film soon afterward though, in the end of 2018 or sometime in 2019. That said, while Black Widow is the best known Marvel female hero, I think she would do poorly in her own film, and even if it were called Black Widow, she would need a lot of support from other characters.
As for DC, I’d expect a Wonder Woman film at the absolute latest in 2018 or 2019. Unlike Marvel, which can continue to do Avengers without Black Widow, DC cannot do Justice League or maintain it’s universe Wonder Woman. She is, without a doubt, their third biggest hero, and the best known female hero. Given that we already have the Man of Steel Franchise going, and we expect a new Batman franchise with Ben Affleck, if they can’t get their third biggest hero her own franchise it would be like if one of Marvel’s tent poles, Iron Man, Thor, or Captain America failed. Sure, Hulk kind of failed, but they’ve taken two shots at him and, like Black Widow, he doesn’t need his own film to work well in the Avengers.
So, here’s how I see it. We know that Man of Steel 2 is hitting in 2016 and Gal Gadot is cast as Wonder Woman in that film, and the rumors are part of the reason it got pushed back was to help set up for Justice League to hit in 2017. Assuming that is true, I suspect they’ll launch into some individual films thereafter, and I’d expect them to try to aim for about one a year or more or they’ll just get forever left in the dust of Marvel. Since they HAVE to establish Wonder Woman, I can’t see them not having hers be one of the first, meaning 2018 or 2019. Of course, they could play it safe and do MoS3 and the first Batman film in those years and push her off to 2020, but I don’t think they can wait that long to give her her own film and have 4 years before the second Justice League film. So I suspect they’ll try to get her and Batman their own films within a year or so of Justice League and use MoS3 to set up JL2 for 2020, and probably try to squeeze in whatever the next most popular character is from the JL lineup they go with, maybe another attempt at Green Lantern or I think more likely Flash, less likely Cyborg, Martian Manhunter, or Aquaman.