Totally different. Without Thor and Cap there is no Avengers period. Instead you wouldve had a buddy movie with IM and the Hulk. Outside of CB nerds (like myself), no one knows who theh hell Hank Pym is and therefore wont miss him.
Im okay with Pym being in an Avengers movie, but there is absolutely no reason to throw him in there before theyve completely tested out the character in a solo movie. By the time the first weekend BO numbers start to come in on Ant-Man, it will be far too late to add him to the movie. Pym is not the same as Cap or Thor or any of the other better known characters. They'll wait and see, as they should. Ill be shocked if they dont.
And comparing Spidey to Ant-Man is ridiculous. For the above reason as well as the fact that Spidey is the most popular superhero in the world next to Batman. And even that is debatable. (Supes might be the most well-known, but not the most popular).
Now if Whedon happens to love the Ant-Man character, maybe he will throw him in there anyway. I just dont see that happening.