Age of Ultron by far, simply because James Spader is going to steal the show. I think Ultron will be one of the great bad guys in CB films.
Got to agree with that, Spader can steal scenes from Shatner (one of the all-time great scene stealers), so no doubt he'll be great as Ultron.
As much as I don't like the Avengers, (sorry Marvel dudes, vastly prefer DC) I have to give credit where credit is due, and Whedon's first Avengers outing is a very, very good action film -and definitely one of the best CBM's. Despite not liking the Avengers at all, I really enjoyed the film( although in terms of Marvel films I think Guardians of the Galaxy was even better ! Now that's a fantastic film. DOFP was entertaining, but forgettable - it doesn't have the re-watch value that Avengers or Guardians has).
Whedon's got great casting, well established characters, the might of Disney behind him and his own prodigious talents - really I'd be utterly amazed if he screwed it up, I just can't see it (although I've been wrong before, especially about George Lucas in similar circumstances). But barring a total brain failure by Whedon Aof U is going to break all kinds of box office records, and unlike the transformers films, be utterly worthy of that success. That's saying a lot for me, as I've never really liked the Avengers (in comics or in their individual films, except the first Iron Man, which is probably the best Marvel origin movie ever, Thor's fun too, Cap, yawn !)
By contrast, I used to love the X-men, my favourite Marvel book for a long time.
But, as someone else pointed out, Singer's big action sequences just aren't as satisfying as they should be - for some reason when the X-men were getting killed in DOFP I didn't really care (and I loved the original DOFP X-comics),
It doesn't matter that I know they're going to win in the end - in LOTR when Gandalf apparently dies in the first film, audiences were stunned (people actually cried, when I saw it in the cinema) even though most knew he was going to be resurrected in the next film.
Yet, when McKellen and co get slaughtered by the Sentinels, it just doesn't have the same impact - which is weird, as they get killed
pretty gruesomely.
Anyway, I've never trusted Singer after he ruined Superman - so on that alone I've got far more faith in Whedon to follow up Avengers with something pretty amazing.
Do we know who's cast as Apocalypse yet ? That would make a big difference, I think, to how well XM:A works out.
Just my opinion though.