That's fairly slippery ground. Let's imagine for a moment that youtube would not have any ads at all. That would not make the copyright infringement any less (they'd be running at an even more massive loss but that's besides the point).

But if they sold the content then it would be direct profiting from piracy and that's a pretty simple affair when it comes to the legal side of things as it allows a much more direct assessment of the damages. No longer can the defendant claim that 'those copies would not have been sold to the viewers' because those copies are sold to the viewers, so the other party has you over a barrel when it comes to proving 'lost income due to reduced sales'.

But whether or not they profited directly from the content or not is actually immaterial to whether or not copyright infringement occurred. It did and charging for access would not make a difference there, income from advertising is just as clearly profiting.

I suspect that if they had charged for the content that youtube would never have gotten as big as it did so clearly the fact that they infringed left, right and center on copyrights and did not charge for the access to that content (and still do...) was what made them successful.

So I'd say yes, it was substantially different, but not in a legal sense.

I don't think youtube would have been alive for both legal reasons (they'd have been hit much harder and earlier) and for reasons related to not being able to achieve the scale they did if they had charged for the content.