Is there any difference between apache+Nginx as reverse proxy AND apache + another server with nginx for static contents?

Question

We use Apache with Nginx(as reverse proxy) for more concurrency level because of the way that Nginx handles static contents and use fewer connections something that Apache lacks.
The question now is that is there any difference between the above scenario and using another server for serving static content (css,js,images,etc) with nginX and your primary server with Apache installed?
In my project there are millions of user with avatar,banner and ofcourse photo gallery. Project is nearly ready, and I want to make sure I'm on the right direction. Which scenario is the best?

EDIT:
What would happen if slow clients cause Apache to keep threads busy for longer than needed in the primary server?

Answer 1

One of the main purposes of nginx behind Apache is to handle slow clients to ensure that Apache doesn't have to keep its threads busy for this.

btw, I think it's relevant to the topic http://www.aosabook.org/en/nginx.html