I'm going to have to go with TV. But all naysayers here me out.
I love me my Avengers and Iron Mans and Dr Stranges. I love seeing them on screen while packed elbow to elbow with other moviegoers trying to ignore the sticky floors and rickety seats (sounds bad, but it's all that ambiance, baby!). Studios can hurl piles of cash at these things too. So they'll, for now, look purtier than anything you can put on CW, Netflix, or (maybe) CBS. And have a wider array of "top shelf" talent to continue bolstering the films' current role as the big kid on the playground.
But they're not the clear front-runner just yet. One issue lies in storytelling. Early after Iron Man, the Fav talked about wanting to try a version of Demon in a Bottle for IM2. Clearly...that's not what we got. And really, how could we ever have gotten that? Aside from the troublesome PR job of having to sell toys from a movie where the main villain is alcoholism, the scope of IM 2 could never do that story justice. At least, not do that story justice AND give us a proper villain AND have cool action sequences AND give your supporting cast their due.
For any who doubt this supposition, I give you...Iron Man 3.
The point is, sometimes with a movie you can't fully or accurately express the finer moments from the comics that we'd love to see on screen. After finishing the Jessica Jones series this morning (hooray, binge!), I can't imagine how even at 2.5 hours of run time Marvel could tell her story as effectively as they did via Netflix. Yes, sure, sure. A Jessica Jones movie could hit all the same notes, provide all the same conclusions. But we'd never get to see characters feel what happens to them. Or grow and change.
I love Avengers AoU, but a cynic could easily argue that the primary character arc of each member is how they're going to beat every new problem into submission. Growth and character development (outside of Tony) is fairly non-existent. It's a basic super hero beat-em-up. A fun one, granted, but still not more than that.
Now imagine making AoU a slow burn that paid off in 12 episodes. Or even six. Think about how much more story you could tell. How much more we could learn about these characters. And how much easier it would be to contrive a thrilling third act that didn't include a homemade asteroid.
Within two years, we've had great success with TV. The Flash, Daredevil, Jessica Jones, iZombie (sorry, Arrow fans; it's a fun show, but it's basically Batman-lite). In that same time, how many as widely celebrated film successes have we had? Which leads us to the final reason TV should be considered the new home of the superhero.
This one is simple and something we've all noticed, I'm sure. You can make up for a bad episode within a week. Even sooner with the Netflix releases. But a bad movie? ONE bad movie can kill a franchise (RIP, Hal). I think it's fair to say that, for someone like me (read: a huge Marvel fan), I'm put off by the oncoming Batman V Supes film, because I've seen Man of Steel. That's the one film by this team I can draw from to get me pumped from BvS. And Man of Steel had Pa Kent sacrifice himself to save the family pet. I mean...not exactly a "with great power..." moment.
So yeah, unless BvS knocks my socks off, I'm going in biased and not likely coming out with that bias largely chained (that's due to Synder, not me; really, I promise). Even the Flash, for which I carry the unsecretest of nerd-chubbies for, had episodes I could have done without. Plastique (episode 5, I think)? That entire episode was simply a set-up to have The Flash run on water for the first time. However, unlike the Synder films, I didn't have to wait three years to see if I'd be disappointed again. And by the next episode or two, it was easy to see where the show was going and convince me to stick around for the rest of them.
Right now, it might be too close to call. Objectively anyway. But very soon, for all these reasons and more, I think small screens take it by a mile.