Come on man... I get believing in Whedon but his directorial experience is less than when those directors started their franchises. Two movies is a ton more experience. Whedon's only movie was released five years ago. I'd feel the same way if it was Shane Black. (and Kiss Kiss Bang Bang is one of my fave films).
Yeah, it depends on what you mean by 'experience', if you are just going by counting up the movies you are not looking at the full picture.
A lot of Jackson's early movies would have been on a far smaller scale than any of the tv work JW has written and directed, same with Nolan's 1st movie 'Following'. 'Memento' was a low budget movie, but I'm not comparing that to JW's tv work, what I am saying is that his work on 'Serenity', involving lots of practical and CG sfx, probably contained more practical prep for a big budget superhero movie than Nolan's films, same with 'Insomnia', which was a remake, and a pretty straight cop thriller.
Nolan had no experience whatsoever with the sfx work needed for a huge superhero movie, before being handed the keys to Batman.
I would put the experience of writing and directing loads of superhero/sci-fi episodes over one movie, or at least it's not 'tons' more to have one more movie under your belt in that context.
You can go on about the major diff between directing a movie and a tv show, but I say you have to measure the scale of the movies with the tv shows(and JW's shows were big, involved productions with large casts and sfx), and also look at the amount of time spent working with actors, in order to get a fairer gauge on 'experience'.
edit: why are there no complaints over on the Spider-man boards about the fact that Marc Webb only has one movie under his belt before being handed the keys to that franchise? He is relative novice, Whedon has been in the biz for more than a decade.